1. A resistance training device comprising: a saddle for a user to sit on; a robot arm adjustable to a length of a limb of the user; a fastener for securing said robot arm to the limb of the user; a controller for controlling a torque of a driving source for driving a joint of said robot arm; and an input operating unit for allowing the user to input a driving condition; said controller being operative in response to said input operating unit receiving from the user as the driving condition a training load and an output direction of a distal end of the limb of the user to be trained to calculate a value of a torque necessary for generating the training load, and causing said robot arm to generate the torque of the calculated value acting in a direction opposite to the output direction, whereby a constant training load is exerted to the user without dependency on a position of the user.