PURPOSE: To observe a lightning current value optically without contacting by making a stroboscope emit light in synchronism with lightning, and converting this light and the flash light of the lightning optoelectrically by photosensors at a proper-distance place and recording the electric signals.

CONSTITUTION: If lightning 2 strikes a lightning observation steel tower 12, the stroboscope 3 fitted to the lower part of the steel tower 2 emits light in synchronism with the lightning. This lightning and the light of the stroboscope 3 form a lightning image 5 and an image 6 on focus glass through a telephoto lens 4 at an observation point. The intensity of the lightning light passed through a slit 7 is converted optoelectrically by a pin photodiode 8 which is fast in response speed. The image 6 of the light of the stroboscope 3, on the other hand, is converted by a pin photodiode 9 optoelectrically. The outputs of the two pin photodiodes are equal in attenuation quantity, so the intensity of the lightning light is observed on the basis of the brightness of the strobo scope 3, so that the lightning current waveform and value are observed.