Japanese is often referred to as a mora-timed language (Ladefoged 1975): the mora has been described as the psychological prosodic unit in the spoken language, and it is the metric unit of traditional poetry (Bloch 1950). However, it is clear that morae are not strictly isochronous units (Beckman 1982). Thus, experimental studies have focused on detecting compensation effects that make average mora durations more equal through the modulation of the inherent duration of the segments involved (Han 1962; Port, Al-Ani, Maeda 1980; Homma 1981; Hoequist 1983a; 1983b; Warner, Arai 2001). Kawahara (2017) used the Corpus of Spontaneous Japanese to verify whether the durational compensation effect within a /CV/ mora occurs in natural speech, in addition to read speech in the lab. He observed a statistically significant compensation effect of /CV/ morae, in which vowel duration tends to vary in response to the duration of the preceding consonant. However, as the same author has pointed out, the compensation is not absolute because there are several linguistic factors that potentially affect segments’ duration profiles. This study will support the idea that moraic isochrony does not occur in spontaneous Japanese by presenting empirical data on how linguistic factors can considerably affect variation in the average duration of morae.