PURPOSE: To realize a simple structure and setting of desired lap by forming the captioned slitter in such a manner as the position of a lapping point can be confirmed promptly and the desired quantity of lapping can be confirmed simply.

CONSTITUTION: Positioning spacers 5, 6, with the same diameter as that of a cutter, and fitted round a lower shaft 3 supported by stands 1, 2, while spacers 8, 9 are fitted round an upper shaft 4 in like manner. At the time of setting a lap, points of contact between the upper spacer 8 and the lower spacer 5 and also between the upper spacer 9 and the lower spacer 6 due to lowering of chocks 10, 11 which support the upper shaft 4, are treated as lap O points between the upper and the lower cutters. The chocks 10, 11 are elevated in order to obtain the desired quantity of lapping under this condition, and a clearance gauge is inserted between the spacers 8, 5 and also between the spacers 9, 6 to confirm if the lap is realized just as a set value. Thus, the desired lap with simple structure can be set.