An output shaft bearing portion which is provided in an extension housing to be attached to an outside of an end wall portion of a casing main body of a transmission casing is lubricated by lubrication oil which is supplied to lubricating portions inside the casing main body. In the end wall portion there is formed an oil guide passage which guides the lubrication oil supplied to the lubricating portions inside the casing main body. Inside the extension housing, there is formed a rib which guides the lubrication oil flown from the oil guide passage, into a position in which the lubrication oil can be scraped by a gear wheel provided inside the extension housing. The lubrication oil scraped by the gear wheel is supplied to the output shaft bearing portion.