Intraindividual variability in positive and negative affect was assessed by the positive affect (Contentment, Joy, Vigor, Love, and Excitement) and negative affect (Depression, Hostility, Anxiety, Agitation, and Social Anxiety) subscales of the state version of the Comprehensive Personality and Affect Scales (COPAS) during a 3-week period. Using the latent state-trait model analysis, which takes both intraindividual variability and interindividual difference into account by controlling measurement error, it was shown that the variability could be measured reliably by the scores of the COPAS. In particular, a total of 56.9% to 63.5% and 48.2% to 60.6% of the reliable interindividual difference in positive and negative affect, respectively,was attributed to the intraindividual variability.