Patent ID: 8651239

Claim:
An oil passage structure for a transmission comprising: a transmission case; a transmission housing chamber formed in the transmission case, the transmission housing chamber reserving oil and housing a transmission mechanism therein; a valve housing chamber formed in the transmission case so as to be disposed above an oil level in the transmission housing chamber, the valve housing chamber reserving oil and housing a valve unit therein; a drain port formed so as to communicate with the valve housing chamber and the transmission housing chamber; and a drain plug detachably disposed in the drain port, wherein oil reserved in the valve housing chamber is drained therefrom by pulling the drain plug out of the drain port, guiding oil from the valve housing chamber to the transmission housing chamber, and reserving oil in the transmission case, wherein the drain plug includes an oil drain passage having a first port and a second port, the first port being open to the valve housing chamber, the second port being open to the transmission housing chamber, and wherein excessive amount of oil in the valve housing chamber is guided to the transmission housing chamber through the oil drain passage such that an oil level in the valve housing chamber is maintained at a constant level.