PURPOSE: To perform high-precise detection of deflection of a tool before machining throughout a range of Φ0.1-Φ8mm of a tool size.

CONSTITUTION: The light emitting element 3 and a light receiving element 4 of a photoelectric conversion element (detector) are arranged in a block 7 placed on a table 8 for a printed board machining device and having a tool insertion recessed part. The elements 3 and 4 are fixed to an approximate specified distance of 20-40mm from the central axis of a tool 9 having a maximum size so that the light reflected by a detecting surface paralleling the central axis of the tool can be received. A control device comprising an amplifier, a computing circuit, and a control panel is connected to the elements 3 and 4. Since a tool is moved to the detector side according to a tool size, a detecting distance is kept at a constant value, the detection precision of a deflection amount is improved, and machining precision is improved.