PURPOSE: To provide the site test device of a ratio differential relay for a transformer in which an emergency stop and the like due to the malfunction of the ratio differential relay are prevented by confirming the systematical trouble of the ratio differential relay.

CONSTITUTION: This site test device is provided with instrument current transformers 15a, 15b which belong to a ratio differential relay 15 for protecting a transformer having a plurality of windings and is provided in the transformer, ternary windings 15a, 15b provided in the instrument current transformer, a test current supplying means 20 for making test currents I1, I2 flow through the ternary windings, current phase adjustment means 21, 22 for adjusting the current value and the phase of the test current and ammeters 26, 27, 29 for detecting a test current and a differential current id flowing through the ratio differential relay. Before an apparatus is operated, secondary currents i1, i2 induced in secondary windings and the operating state of the ratio differential relay are referred while a phase-adjustable test current is made to flow through the ternary winding.