PURPOSE: To surely prevent malfunction by providing a resistor having an electric connection means to a main body device on a component removable from the main body, measuring the resistance value with a resistance value measuring circuit, and allowing the operation of the main body device when this value is within the preset range.

CONSTITUTION: A resistor 4 with the preset resistance value is provided in an external optical component 2. When the external optical component 2 is correctly fitted to a laser device main body 1 and fixed with a fixing ring screw 3, the resistor 4 is connected to the resistance value measuring circuit of the laser device main body 1 via electrical contacts 5 and 6 and an electric wire for measurement 7. When the measured value is confirmed to be within the preset range by this resistance value measuring circuit, the interlock is released, laser rays are emitted from the laser device main body 1. On the other hand, when components other than the external optical component 2 are fitted or brought into contact with the laser device main body 1, the measured value by the resistance value measuring circuit is outside the preset range, thus the interlock is operated, and no laser ray is emitted. A dangerous situation can be thereby avoided.