This study developed and examined the validity of Japanese versions of the Daydream Frequency Scale (DDFS) and the Mind Wandering Questionnaire (MWQ), which measures propensity for spontaneous thoughts and mind wandering, respectively. In Study 1, we translated the items of the DDFS and the MWQ into Japanese and verified their validity. In Study 2, we confirmed the correlation of both scales with mind wandering, as measured by thought sampling during an attention-demanding task. These two studies revealed a dissociation between the properties of the scales; while DDFS reflects propensity for spontaneous thoughts, MWQ specifically reflects propensity for mind wandering. We discuss the usefulness of the DDFS and the MWQ for studying the psychological functions of spontaneous thoughts and mind wandering.