Most of the tasks that people perform daily require focused attention, but our minds still sometimes wander. This phenomenon is known as mind wandering. Some studies have shown a negative correlation between mind wandering and performance in tasks. The aim of this research was to check the connection between the performance in the working memory capacity task and the frequency of mind wandering depending on the difficulty of the task. Besides that, we wanted to examine the relationship between subjective and objective measure of mind wandering. 74 participants between the ages of 18 and 29 took part in the research. Participants completed the Mind Wandering Questionnaire (MWQ), the aOspan task and 1- (N=37) or 3-back (N=36) task depending on the assigned group. During the n-back tasks, they were asked on three occasions if their mind was wandering at the moment. A negative correlation between the difficulty of the task and the frequency of mind wandering was established, such that participants in the situation of 3-back task reported less frequent mind wandering than those in the situation of 1-back task. The mediation analysis of performance in the easy and difficult N-back task on the association between working memory span and mind wandering was not justified to implement. A significant positive correlation between the MWQ questionnaire and the observed amount of mind wandering during the N-back task was obtained. Unambiguous results about the nature of the role of task difficulty in the association between working memory span and mind wandering have not been obtained.