In the step-up DC-DC converter built as a trial, a single phase thyristor bridge furnished with commutating capacitors by means of series-diode-system is applied as an inverter, and a full-wave type 4-fold voltage rectifier is used as a step-up equipment. The constitution and manipulation of this apparatus is comparatively simple, and only four chemical condensers are required as the main condensers of the step-up equipment part. In order to prevent abnormal rise of the commutation voltage and the output voltage at the case of no load or light load, the feed-back diode bridge is required too. In this paper, the constitution and an outline of the operation of this converter, and the performance characteristics of it are chiefly described, and thus the optimum condition of the operation is found, and furthermore the points to improve in future are investigated. It is introduced that when the capacitance of a main condenser is 3200μF and the input DC voltage is 40∿50volts, for the output power of 1∿1.4KW, about 4-fold output DC voltage is obtained, the efficiency is 75∿81%, the pulsation factor is 2.5∿0.55%, the voltage regulation is 40∿25%, and then the optimum driving frequency of the inverter is 200∿300Hz.