line of the blade receiving face 11 whereby the one end 13 a of the swinging member 13 is allowed to be easily placed substantially parallel to the blade receiving face 11 as described above although the one end 13 a of the swinging member 13 is positioned between the upper face 10 and the bottom face 9 of the blade holding portion 8 the other end 13 b is not positioned between the upper face 10 and the bottom face 9 and has a shape which elongates so as to separate from the blade receiving face 11 or radially outward separate from the plunger 1 as shown in fig4 and 6 a hole la having an inner wall face which is substantially flush with the face of the groove 11 a is formed in the plunger 1 when the blade 7 is attached an end of the attachment portion 7 a of the blade is positioned in the hole 1 a of the plunger 1 in the housing 2 a lever 15 which serves as an operation member and which has a swing fulcrum 15 c that elongates in a direction substantially identical with the longitudinal direction of the plunger 1 is disposed on the housing 2 so as to cover the blade holding portion 8 in the lever 15 an abutting portion 15 b which can abut against the other end 13 b of the swinging member 13 is disposed at a position close to the swing fulcrum 15 c and an operation portion 15 a is formed in a place which is remote from the abutting portion 15 b the lever.