The present invention provides a preventive or therapeutic agent for bruxism or bruxism-related diseases that contains as an active ingredient at least one selected from the group consisting of proton pump inhibitors, histamine H.sub.2 receptor antagonists, and acid pump antagonists. Examples of the proton pump inhibitors include rabeprazole, omeprazole, esomeprazole, lansoprazole, pantoprazole, or tenatoprazole, or salts thereof, or hydrates thereof.