To provide a safe cutter knife of which the cutter is automatically housed into a case after use.

Both flanks of a case 1 are provided with long holes 2. A movable pusher 3 is fitted into the long hole on one side and a movable stopper 5 attached with a knob 4 is fitted into the long hole on the opposite side. The cutter 6 is fixed to the pusher 3 and the inside of the case 1 is provided with a tension spring 7 of the cutter 6 detained to the rear end of the case 1 and part of the pusher 3. The pusher 3 and the cutter 6 are integral and are slid by bring guided with a cutter guide 8 disposed at the case 1.