An oil passage structure for a transmission includes a transmission case having a transmission housing chamber for reserving working oil and housing a continuously variable transmission mechanism therein, and a valve housing chamber for reserving the working oil and housing a valve unit therein. Further, the valve housing chamber housing the valve unit is disposed above an oil level in the transmission housing chamber. Furthermore, the valve housing chamber and the transmission housing chamber communicate with each other through a drain port closed with a drain plug. In demounting of the valve unit, the drain plug is pulled out to open the drain port. Hereby, the working oil can be transferred from the valve housing chamber to the transmission housing chamber, and the valve unit can be demounted without draining the working oil from the transmission case.