Abstract: A constant-pressure mechanism includes: a pressure transmission member being linearly movable in a predetermined direction and being urged by an urging member; a rotary member being rotatable around a central shaft located in a case receiving the pressure transmission member; a follower member supported to the case so as to be movable linearly; and a displacement conversion mechanism interposed among the pressure transmission member, the rotary member and the follower member. The displacement conversion mechanism has a first link arm pin-connected between one end portion of the rotary member and the pressure transmission member, and a second link arm pin-connected between the other end portion of the rotary member and the follower member.