Abstract: A thyristor is shunted by a metal oxide varistor in series with switching means which switches abruptly from a normal high-resistance state to a low-resistance, current conducting state when the thyristor is subjected to a transient surge of reverse bias voltage the magnitude of which is higher than a predetermined repetitive peak magnitude and is nearly equal to the level at which the thyristor turns on in an avalanche breakdown mode. The voltage-current characteristic of the varistor is coordinated with the reverse breakdown characteristic of the thyristor so that surge current will divide therebetween in such proportions that the maximum reverse current flowing through the thyristor is lower than a predetermined critical magnitude which can damage the same, whereby the thyristor is able safely to participate in the surge-energy dissipating process.