To provide a supplementary implement for Seiza (formal way of sitting on the floor in the formal posture) which can be used inconspicuously in a ceremony or the like, regarding the supplementary implement for the seiza used for sitting in the seiza posture with ease.

A strip cloth is fitted to a supporting implement such as a small stool that supports the upper body with the buttocks placed thereon. An internal belt to be worn on the underwear of a waist part is prepared separately. The opposite end parts of the strip cloth are held respectively by the front and rear parts of the internal belt worn on the waist part, so that the supporting implement can be retained at a crotch. Since all of these members are hidden under an upper garment or a ceremonial garment, they are inconspicuous in a ceremonial site and the seiza can be performed easily with the help of the supporting implement.