A stretching structure of a stretching material in a chair in which the ratio of the rear frame of a backrest to the outline of the chair is small, design is smart, weight is reduced, the number of parts is reduced, and assemblability is improved and the backrest of the chair. In the chair having the backrest formed by stretching the stretching material on the front surface of the rear frame, the rear frame comprises a front frame to which the peripheral edge part of the stretching material is fixed and an upper reinforcement frame rod. The laterally facing upper reinforcement frame rod is connected at its both ends to both ends of the laterally facing upper frame rod at the top of the front frame with the center part of the upper reinforcement frame rod separated backward from the upper frame rod.