A user repeats a right and left swaying motion while holding a movable body in which a resistance force is generated by tension of an elastic member, hydraulic pressure, pneumatic pressure, or the like, between his/her two legs, thereby efficiently strengthening, particularly, abdominal muscles, such as rectus muscles of an abdomen, external abdominal oblique muscles, and transverse abdominal muscles, erector muscles of a spine, or iliopsoas muscles, and strengthening various kinds of abdominal and lumbar muscles with an easy motion without laying burden on a waist, unlike the motion of making a user's upper body stand or lie down, which is conventionally seen. A machine trains various kinds of abdominal and lumbar muscles, by a movable body being held between the user's two legs and is swingably journalled to a base plate. The base plate is pivotable with respect to a seating plate on which a user sits, and a resistance force is loaded to the movable body in a swinging direction, and an elastic member, such as rubber or a spring, or a member using hydraulic pressure, is interposed between the movable body and the base plate.