PURPOSE: To high accurately and high efficiently operate a machining device and easily perform its handling, by rotating a cylinder and moving a tool in an axial direction so as to detect a relative position between both the cylinder and the tool and control a difference between said relative position and the preset value to zero, in the case of the machining device of the cylinder for a video tape recorder.

CONSTITUTION: Data for forming a head shape on a cylinder are stored in a random access memory (RAM) 35, operating a control circuit 30. In this way, the cylinder 13 is machined to be cut by rotating the cylinder itself and moving a tool in the axial direction of the cyliner. Here a rotary angle of the cylinder detected by a rotary encoder 31 and a moving position of the tool detected by a linear encoder 32 are input to the control circuit 30. The control circuit 30 compares in its comparator 36 a linear encoder signal sampled through a sample hold circuit 34 with a reference signal from the RAM 35, and a motor driving circuit 38 controls a motor M so that a difference between the compared both signals may become zero. In this way, a machining device, being easily handled, can be high accurately and high efficiently opened.