A transmission has a movable transmission part movably connected to a fixed transmission part via a shaft to undergo pivotal movement relative to the fixed transmission part about an axis of the shaft. Each of the transmission parts has a hollow cylindrical connecting portion through which the shaft extends. One of the connecting portion of the movable transmission part and the connecting portion of the fixed transmission part is received in the other of the connecting portions with a single seal member disposed therebetween so that an internal space of the fixed transmission part and an internal space of the movable transmission part communicate with each other to thereby form a single hermetically-sealed space for holding therein a lubricating oil.