A phase adjusting apparatus for use in an assembling machine for assembling first and second workpieces including a movable frame which is arranged to move into and out of the space formed between the two workpieces. Prior to coupling the two workpieces, the movable frame is moved to enter the space and a first movable table and a second movable table on the movable frame are used to adjust the phase of a rotary body on the first workpiece and the phase of a rotary shaft on the second workpiece to another. After removing the movable frame from the space, the two workpieces are then brought together so that the rotary body and the rotary shaft are coupled to each other.