RETARDER

[Problems to be Solved] In a retarder equipped with an induction motor, eliminating the requirement of a rotation speed sensor for detecting a rotation speed of the induction motor. [Means for Solving the Problems] In a retarder (10) formed of an induction motor (11) coupled to an output shaft (43) of a transmission (42) and a control unit (12) for controlling the induction motor (11), the control unit (12) obtains information detected by a rotation speed sensor for an engine (40) and information indicating a gear state of the transmission (42) from a vehicle (1) side (outside of the retarder (10)), derives a rotation speed of the induction motor (11) based on these information, and controls the induction motor (11) based on the derived rotation speed.