A lancet having a slider with a puncture needle holding mechanism at one end thereof, and having a cam ring with a continuous cam groove, rotatable about a support shaft, and with a cam ring claw and an anti-return claw restricting the rotation. The lancet also having a ring spring applying a force to rotate the cam ring, having a rotatable stopper arm holding and releasing the rotation of the cam ring, and having a rotatable ratchet restricting the direction of rotation of the cam ring. A puncture needle unit including a puncture needle body integrally molded with a protrusion fitted to the lancet, a rotation stop rib, a puncture needle, and a puncture needle cap lightly pressed into the puncture needle body.