PURPOSE: To set the power supply voltage of a trigger circuit at a low level and to increase the reliability, by having the emission of light of a number of light emitting elements connected in series to each other with a boosted signal and triggering a number of optical trigger syristors connected in series to each other with the output of light emission.

CONSTITUTION: When a transistor 4 is turned on with the output of a control circuit 5, a current iL' is supplied to a pulse transformer 8 from a power supply 1 via a current limiting resistance 2. At the same time, a current iL" flows to a group of light emitting elements 3 connected to the secondary side of the transformer 8. As the transformer 8 has a 1:n winding ratio, the group of elements 3 can be driven satisfactorily with the boosted voltage of a high level although the voltage of the power supply 1 is at a low level. The output of light emission of the element 3 is applied to an optical trigger thyristor 7 via an optical fiber 6 to trigger the thyristor 7.