<p>A hydraulic transmission coupling apparatus which is used instead of a center differnetial or a viscous coupling being usually mounted between a front wheel drive shaft (1) and a rear wheel drive shaft (2) of a four-wheel drive vehicle, and transmits driving force existing between the two shafts. The hydraulic transmission coupling apparatus is provided with a vane-type hydraulic pump (3) which generates hydraulic pressure corresponding to a relative rotation speed difference of the two drive shafts (1,2) and a variable throttle valve (5) being disposed in the passage (48) connecting a discharge port of the hydraulic pump (3) with a tank (T), whose opening area varies according to the hydraulic pressure generated by the hydraulic pump (3), thereby driving force to be transmitted being increased or decreased steeply as the rotation speed difference becomes larger.</p>
