An operating shaft rises and falls by abutting against a cam surface of a cam rotatably built into the upper part of a stem. A swivel-preventing plate part is fixed to the lower end of the operating shaft. An engagement member is fixed to a support yoke and regulates the swiveling by engaging with the swivel-preventing plate part. A brake shoe part is fixed to the lower part of the swivel-preventing plate part and brakes by abutting against a treat surface of a wheel when the operating shaft falls. A link cam causes a link cam to pivot and causes a brake plate to rise and fall by connected to the brake shoe part. A protruding part for braking, by displacement of the brake plate, is capable of engaging with and disengaging from an annular engagement receiving part formed in the inner periphery of the wheel.