PURPOSE: To provide the incision depth monitoring device by which a measurement error of incision depth of an incised groove can be reduced remarkably.

CONSTITUTION: The monitoring device is provided with a hand piece 3 for irradiating a part to be irradiated of an object to be incised with a laser beam from the tip, a transducer which is provided in the hand piece 3 and detects an acoustic wave which is generated in the part to be irradiated at the time of irradiating the part to be irradiated with the laser beam and propagated in the air, and an arithmetic means for calculating incision depth of the part to be irradiated based on a detection signal outputted from the transducer. Also, the hand piece 3 is provided with an assist gas introducing tube 160 and an opening part 161 for reducing a measurement error of incision depth by eliminating a scattered matter 9 generated from the part to be irradiated at the time of irradiating the part to be irradiated with the laser beam.