Abstract: The present study investigates several types of common diachronic meaning shifts through detailed synchronic and diachronic analyses of -te-shimau , a marker of completion, lack of agency and negativity in present-day Japanese (grammaticalized from the verb shimau 'to put away' or 'finish'), which appears to undergo the following five types of shifts: (1) from spatial meaning to temporal meaning; (2) from 'intentional' meaning to 'non-intentional' meaning; (3) toward subjective meaning ( subjectification); (4) toward affective meaning (affectivization ); and (5) toward intersubjective meaning (intersubjectification ). The study focuses on subjectification. Subjectification has been claimed to be unidirectional in nature. That is, the meaning of linguistic items or constructions tends to become increasingly situated in speaker's subjective belief state or attitude toward described content, rather than going in the opposite direction (Traugott 1989, 1995, Traugott and Dasher 2002, etc.). The purpose of the present dissertation is to elucidate the nature of subjectivity from the perspectives of cognitive semantics (Talmy 2000a, b) and historical semantics and pragmatics (Traugott 1989, 2003, Traugott and Dasher 2002) mainly in terms of: the factors involved in the types of diachronic shifts mentioned above; the relationships among the shifts; and the cognitive implications of the semantic shifts. The synchronic analysis provides semantic descriptions of -te-shimau , starting from a bottom-up characterization with twelve so-called alternative semantic components, and includes an empirical study of how native speakers interpret the usage of -te-shimau in conversation data which illustrates how different functions of the marker often overlap in a single instance when subjectivity and/or intersubjectivity are involved. The patterns of overlapping, or layering, reflect certain reasoning patterns or communicative strategies that are often culture-dependent. Furthermore, the semantics of -te-shimau can be effectively characterized using force dynamics and perspective (Talmy 1988a, b). The diachronic analysis attempts to sort out the types of factors that need to be considered for characterizing semantic shifts and then looks at the usage of the marker in different historical stages from early-modern to present-day Japanese, starting with a characterization of the lexical meaning in the pre-grammaticalization stage, which has not previously been investigated sufficiently. I show that the original meaning of lexical shimau is best characterized as a hybrid notion between spatial and temporal domains involving “inaccessibility”. The initial grammaticalization stage shows evidence of the complexity of the interaction between the formal and the semantic sides of the transition from shimau to -te-shimau. In the further grammaticalization stage, we see increased use of -te-shimau with third person subjects in combination with an increase of non-agentive usage, as well as a significant increase of uses in which -te-shimau indicates lack of agency in the speakers' own actions, or in state changes in which he/she is involved. The dissertation shows that the marker -te-shimau is best characterized in terms of layering of functions, the coexistence of senses with different shades of (inter)subjectivity. The dissertation also suggests that the meaning shifts can be interrelated to each other in complex ways. Furthermore, the study highlights certain cognitive implications grounded in the development of polysemy of (-te)-shimau and emphasizes the effectiveness of schematic characterizations as well as a historical semantic approach.