PURPOSE: To obtain a rigid device with respect to mechanical vibration, by taking out monitoring light for detecting the energy intensity of a light source through a scanning window provided at a part of a cover, which has an emitting port, from which the output light of the light source is projected on a member to be measured.

CONSTITUTION: A cover 5 shields a light source 4 and has an emitting port 6 for projecting laser light l1 on a member to be measured (not shown). Monitoring light l2 from the light source 4 leaks from a scanning window 7 along the same axis as that of the light l1. A cover member 8 is provided at the monitoring-side end part of the cover 5 so as to cover the window 7 as a unitary body. A light energy detector 9 is arranged as a unitary body. When the energy of the light l2 is kept at a constant value, l1 having the accurate energy intensity is projected on the member to be measured. Since the window 7 and the detector 9 can be positioned and fixed as a unitary body with respect to the cover 5, the device becomes rigid against mechanical vibration.