Abstract: A photodetection device for accommodating photodetection elements and for detecting a photodetecting position, comprising a base, a transparent cylindrical case fixed on the base and having a closed upper end, guide grooves formed with the case, and substrates with electronic components, wherein the substrates are engaged and held in the guide grooves.

Abstract: A laser beam detecting device is disclosed for detecting a laser beam which is emitted from a laser beam generator and which forms a reference plane. A power supply for a laser beam sensor used in the laser beam detecting device is turned OFF automatically when a construction machine with the laser beam detecting device attached thereto is not in operation, while upon operation of the construction machine, the resulting vibration is detected by means of a vibration sensor and the power supply for the beam sensor is turned ON. By so doing, there is attained a superior power saving characteristic and it is possible to effect operation over a long time even with use of cells or a battery.