PURPOSE: To enhance space efficiency and economical effect, in a high voltage thyristor converter constituted by stacking thyristor valves in a plurality of stages, by performing the diagnosis of an element obstacle by a reduced number of microcomputers.

CONSTITUTION: The voltage detecting sigansl au1WauN of thyristor valves U1 can be taken in a time sharing manner by a microcomputer MC and, after one or more logical data of voltage detection are taken in at every each value to be stored in an RAM circuit 22, the obstacle discrimination program preliminarily stored in an ROM circuit 24 is practiced to perform the discrimination of the element obstacles of the valves U1, X1. When the obstacle of a thyristor element is discovered, obstacle data such as an obstacle signal, the number of obstacles or the like are sent out as an output signal C from an I/O circuit 23. By this mechanism, the diagnosis of the element obstacle can be performed by a reduced number of the microcomputers and the enhancement in space efficiency and economical effect can be achieved.