To provide a measuring instrument which can measure a finished cutting dimension in the direction from right to left in a printed book on a three-way cutting machine.

The measuring instrument comprises a depth gauge 30 having a slidable main scale 31, a supporting column 35 on which the depth gauge 30 is fixed, and a base stand 33 for supporting the supporting column 35. The finished cutting dimension in the direction from right to left in the printed book can be accurately measured on the three-way cutting machine by bringing the main scale 31 of the depth gauge 30 into contact with the cutting surface of the edge knife of the three-way cutting machine in the state that the surface 34a of the contact portion 34 of the base stand 33 has been brought into close contact with the stopper of the three-way cutting machine, and by reading the depth gauge 30.