PURPOSE: To make it possible to obtain exactly a total actual machining time by detecting the state of at least one of a work and a machine tool being ready to operate with the other and then by adding their contact times detected from changes in the electrical resistance between them.

CONSTITUTION: A light receiver 13 receives light from a projector 12 per rotation of a spindle 3, and outputs the signal, which, after being converted through a filter circuit 17, enters into the base of a transistor 52. In this way, only when the spindle 3 is rotating, the transistor 52 is turned on and an excitation coil 54A of a relay 54 is energized to close a contact point 54B which is usually opened. On the other hand, when a drill 5 comes in contact with a work W, the electrical resistance decreases, and therefore a voltage converter 34 outputs the signal. This signal is compared in an ope-amp 38 and transformed into a rectangular wave form and, after being converted through a filter circuit 43, is fed to the base of a transistor 55. In this way, only when the drill is not in contact with the work W, the transistor 55 is turned on, and an excitation coil 57A of a relay 57 is energized to open a contact point 57B which is usually closed. Thus the actual machining times are added.