A power regeneration system of a work vehicle includes: a first generator (12) and a second generator (31) that are driven by an engine (11); a first electric circuit (C1) for supplying electric power to traveling motors (10L, 10R); a second electric circuit (C2) for supplying electric power to an auxiliary device (35); a voltage step down converter (21) providing electric power from the first electric circuit to the second electric circuit; and a controller (51) that determines whether a travel mode of the work vehicle is a powering mode or a regeneration mode in order to control driving of the voltage step down converter. If the travel mode is the regeneration mode, the controller provides the regenerative power from the first electric circuit through the voltage step down converter to the second electric circuit, in order to drive the auxiliary device with the regenerative power.