A controller used for irradiating a laser beam to an object from a machining head at a reference gap position is provided. The controller includes a gap sensor for detecting an amount of gap between the machining head and the object, a gap position command calculation part for producing a gap position command, a servo mechanism for driving the machining head to the reference gap position, a servo position deviation reading part for reading an amount of position deviation of the servo mechanism, a position gain calculation part for calculating a corrected position gain of the servo mechanism, based on the amount of position deviation of the servo mechanism and a position gain replacing part for replacing the position gain of the servo mechanism with the corrected position gain.