Abstract: A head-rest vertical adjustment device is disclosed which is provided with a lock mechanism to restrict the vertical movements of a head-rest stay in a guide cylinder into which the head-rest stay is inserted in a manner that it is free to move vertically. This lock mechanism comprises a lock spring of a wire material to be engaged into an engagement groove formed in the head-rest stay from a slit formed in the guide cylinder, and an operation lever to release the lock spring from its locked position. First ends of both the lock spring and operation lever are rotatably mounted onto the same projection provided in the guide cylinder and second ends of these two members are integrally connected to each other.