In a hybrid transmission and its assembling method, a differential unit is constituted by a single pinion planetary gear set and a double pinion planetary gear set holding a carrier in common to the single planetary gear set and having five rotary members from among which, when revolution states of two rotary members are determined, those of the other rotary members are determined, an input end from a prime mover is coupled to a ring gear of the double pinion planetary gear set, an output end to a drive system that is coupled to the common carrier, a first motor/generator that is coupled to a sun gear of the double pinion planetary gear set, a second motor/generator that is coupled to a sun gear of the single pinion planetary gear set, and a brake enabled to brake a ring gear of the single pinion planetary gear set.