The ta-form of dynamic verbs (V-ta) has two negative forms: V-nakatta 'did not V' and V-tei-nai 'has not V-ed'. This is normally considered as evidence supporting the claim that V-ta is semantically ambiguous: V-ta whose negative form is V-nakatta expresses past perfective, while V-ta whose negative form is V-tei-nai expresses present perfect. I show that this claim is incorrect for the following reasons. (i) The meaning of V-ta, which is said to express present perfect, is different from that of V-teiru as a present perfect form in Japanese. (ii) It is implausible to determine the meaning of V-ta based on the meaning of the corresponding negative form. (iii) The opposition of V-ta (affirmative) / V-tei-nai (negative) is realized at a pragmatic or contextual level rather than a semantic or logical level and therefore it cannot be evidence for the semantic ambiguity of V-ta. (iv) It is the semantic property of V-ta which can be called "high dynamicity of event description" that establishes the contextual opposition between V-ta (affirmative) and V-tei-nai (negative). The so-called semantic ambiguity of V-ta is merely the difference between sentences which specify whether or not the event occurred and those which specify whether or not the event belongs to the past.