Our main objective in this paper is to evaluate the possible association between interpersonal mistrust and unhappiness among Japanese people. Based on cross-sectional data for the Japanese general population from the Asia Barometer Survey (2003-2006), we analyzed the relationship between interpersonal mistrust and unhappiness using a logistic regression, adjusted for age, gender, marital status, income, education, occupation, religious belief, and self-rated health. In a total of 2685 participants (mean age, 42.7 years; 47.5% men), 204 (7.6%) were classified as unhappy. For the questionnaire items involving mistrust, 1490 (55.5%) participants reported that they "can't be too careful in dealing with people", and 1642 (61.2%) participants reported that "people mostly look out for themselves". In a multivariable-adjusted model using the average score based on these 2 questions, interpersonal mistrust was associated significantly with unhappiness, with an odds-ratio of 2.06 (95% CI, 1.25 to 3.38). Other features associated with unhappiness included: age 50-59 years, marital status of single, divorced, separated, or widowed, low income, mid-education, and poor self-rated health. Gender, occupation, and religious belief were not associated with unhappiness. In conclusion, interpersonal mistrust is associated significantly with unhappiness among Japanese people. Public policies restoring interpersonal trust are needed to promote happiness among the Japanese.