To easily change oil for automatic transmission by a method wherein oil lines are changed automatically even though a supply line and a discharge line are erroneously connected in the automatic oil replacing device to be connected respectively to a filling port and a discharge port of the automatic transmission.

The supply line (16) and the discharge line (18) of the automatic oil replacing device are formed respectively in a double line, and an oil line changing part (20) having two-way solenoid valves (22, 24, 26, 28), a solenoid valve (32), a used oil discharge tank (34), a motor pump (46), a new oil supply tank (44), a solenoid valve (48) and a pressure gauge (49), are provided respectively at each of the double lines.