An assisting instrument allows a person to effectively train their core muscles by movements in twisting directions or one leg raises, and allows the twisting movements to be made in a prone posture. A first body part is comprised of a rectangular parallelepiped part and an integrated semicircle pillar section. When the assisting instrument is used during workouts in the prone posture, the person's two anterior superior iliac spines and pubis part are protected by a first pad, a second pad, and a third pad, respectively, and their abdomen is fitted into a groove part. When the person raises one of their legs, the semicircle pillar section rolls to the opposite side. As a result of holding their body to prevent the rolling and prevent their body from being thrown out of the instrument, the person puts their strength in their core muscles which can thus be trained.