PURPOSE: To eliminate the need of thread cutting work, to simplify various mechanical work on knife manufacture and to reduce the cost by attaining a clasp knife for surely fixing a knife blade operated by a push button operation applicable to both single blade and two blades to three fixing positions.

CONSTITUTION: In order to open the blade 24 so as to move to a second fixing position, a push button projection part 32 for forming a first trunnion is pressed in an axial direction, a spring 36 is compressed and a square plate material 34 is sufficiently moved, pressed into a square opening part 26 and completely put out from a square recessed part 30. In the meantime, a member 31 is turned to the position and the blade 24 becomes movable to the other positions of the three positions separated from the position shown in the figure for 90°. When it reaches at a new fixing position, the square plate part 34 is affected by the spring 36 and gnawed into the square recessed part 30 and the blade 24 is surely fixed again. The member 31 is then returned to the position shown in the figure. While the blade 24 is turned among the three positions to be fixed, the member 31 is turned together with the blade to handle plate materials 20 and 21 and the members 32 and 33 function as a trunnion.