direct to the predetermined value at every emission and therefore the average value of the pulse energies directs to a predetermined value in fig4 the emission period is shortened from time t3 however due to the operation of the switch 21 the time ts for which the small signal u03b4s is supplied to the integrating circuit 9 is independent of the emission period and therefore the pulse energy can be kept constant as well as before time t3 although in this embodiment the peak value of the pulse strength is held it is also appropriate to hold the integrated value of the pulse waveform further it is appropriate to use a sample and hold circuit which is a general ic circuit instead of the peak detector 8 in addition the divider 20 is not necessarily provided independently and it is also appropriate to reduce the gain of the peak detector 8 or the comparator 7 moreover the synchronous circuit 3 is not limited to the description it is also appropriate to detect the emission in conjunction with the power supply 2 the light emitting section 1 and the photodetector 5 a second embodiment of this invention will be described hereinbelow with reference to fig5 in fig5 a pulsed light source of this embodiment similarly includes.