The present paper aims to study the auditory processing of foreign languages, especially English, by native speakers of Japanese. Several pieces of research have shown that listeners use assimilated segments to facilitate online auditory processing (Gow, 2003; Otake et al., 1996). Shoemaker (2014) states, however, that it is not only assimilation viable phonetic context but the much lower level of phonetic information like duration that plays a vital role in processing using assimilated segments. Since Sato (1993) revealed that the relative duration of syllable-final consonants in Japanese was 1.54 times longer than those of English, it has been expected that an assimilated English final nasal is not long enough to provoke facilitation effect for Japanese listeners but that the effect can be triggered if the assimilated final nasal is modified to be as long as the Japanese one. The present experimental study investigates whether the lengthened assimilated English final nasal of a pre-target can provoke Japanese listeners’ facilitation effect or not. The result indicates that the modified longer assimilated final nasal facilitates Japanese listeners’ language processing. This result is apparent evidence that Japanese listeners process English sounds based on their knowledge of Japanese; syllable-final segmental duration in this case.