To provide an oil level adjusting device of an automatic transmission, eliminating difference in oil level due to the temperature of oil to surely prevent discharging oil with air by a breather plug to the outside while suppressing the rotational resistance of a final ring, and enough supplying oil for lubrication between the gears.

In this oil level adjusting device of the automatic transmission, an oil pan 2 is provided with a flat plate-like split pan 23 for vertically splitting the content volume into an upper volume part 21 and a lower volume part 22. The split pan 23 is provided with an inlet pipe 24 erected for introducing oil exceeding the oil level X at an ordinary oil temperature out of the oil of the upper volume part 21 stored on the split pan 23 into the lower volume part 22. An oil suction port 42a of a suction pipe 42 for sucking oil by an oil pump is opened to the lower part side in the lower volume part 22.