hammer 30 and send a signal corresponding to the number of the protrusions 81 to the control unit since a stroke length of the piston rod 51 of the second hydraulic cylinder 50 is set to be longer than the spacing between the upper stoppers 68 a and the lower stoppers 68 b the piston rod 51 is further raised even after the guide protrusions 63 of the plunger unit 61 have been caught by the upper stoppers 68 a more specifically since the piston rod 51 passes through the connector 65 slidably fitted in the guide hole 64 b of the plunger unit 61 and is connected to the actuating rod 45 of the pusher unit 43 via the connecting pipe 48 the piston rod 51 can be further raised until the actuating rod 45 is raised to the top of the casing 41 and thus caught by the cap 44 as shown in fig1 d in this way since the actuating rod 45 is further raised after the upward movement of the hammer 30 is stopped the dog 47 in the casing 41 is raised with respect to the push blocks 42 thereby allowing the drop head 46 to be released from the dog 47 at this point since the pressing force applied to the push blocks 42 which are in state of pressing the inner surface of the hammer 30 radially and outwardly is removed the hammer 30 falls by its own weight and thus impacts against the anvil 91 thereby.