To easily perform setting of a width and adjusting of tension in the case of using an expander with one extensible member.

The expander is composed of at least two grip members and the one extensible member. At each of the two grip members, one or more gripping parts and an extensible member holding part having a surface for holding the extensible member and formed of a member of a shape of a plate, a bar or the like are formed. In at least one of the two grip members, the width of the extensible member holding part is ≥4 cm at its widest part without regard to the number of members forming the part. The extensible member is formed of a member which is extensible and flexible and has a shape of a cord, a belt, a chain or the like having some length. Then, the expander is used by hanging or winding the extensible member to the surface of the extensible member holding part.

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