A jig is used for positioning a retaining ring that retains a seal ring in an inner periphery of a blade ring relative to the blade ring that constitutes a gas passage of a turbine. The jig includes a base that determines a measurement point on the blade ring, a shaft arranged displaceably relative to the base, a probe that is fixed to the shaft and determines a measurement point on the retaining ring, and a measuring unit that is arranged in the base and measures a displacement amount of the shaft relative to the base.