An easily operable lock device for a movable member of a chair, the lock device being adapted such that a person sitting on the chair can select lock or unlock position by operating operation means with a load, such as the weight of the person, acting on the movable member and, when the load is removed after the selection of the lock or unlock position, lock or unlock operation is executed. A lock pin provided at a spring receiving member functioning as a movable member, is selectively engaged with and disengaged from engagement holes formed in a reinforcement plate functioning as a support member. The lock device has biasing means for biasing the lock pin toward the lock position when an operation member moved by the operation means is set at a lock reservation position and biasing the lock pin toward the unlock position when the operation member is set at an unlock reservation position.