PURPOSE: To prevent endless enlargement of a beam diameter by repeating reflection by processing the outside peripheral surface of a cylinder so as to form a confocal type mirror with a gyro main body.

CONSTITUTION: The optical reflection surface whose outside periphery is the cylindrical surface with transparent optical material and the incident and outgoing surface of laser beams are provided to the gyro main body 1. Then, a beam splitter 2 is arranged corresponding to the incident and outgoing surface. Further, a laser oscillator 3 projects the laser beams on the inside of the main body 1 in the clockwise direction and the anti-clockwise direction via the splitter 2. Then, detectors 4 and 5 detect a phase difference between the laser beams which are turned reflectively and outgone by way of the clockwise direction and the laser beams which are turned reflectively and outgone by way of counter clockwise direction in the main body 1. The focus distance of the reflection surface of the outside periphery of the main body is almost the same as the distance to the next surface where the reflected light of the laser beams attains. In this way, the outside peripheral surface of the cylinder is processed so as to form the confocal mirror and the endless enlargement of the beam diameter is prevented even if the reflection is repeated.