1. A resistance training device comprising: a seat on which a user sits; a robot arm adjustable to a length of a limb of the user and having a first link connected to a first joint axle at a first location corresponding to a first joint of the user and connected to a second joint axle at a second location, different from the first location, corresponding to a second joint of the user, a second link connected to the first link at the second joint axle, and a first driving source and a second driving source respectively connected to the first joint axle and the second joint axle for respectively rotating the first and second links about the first and second joint axles; at least one fastener for securing said robot arm to the limb of the user; joint angle sensors for sensing respective joint angles of said first and second joint axles; an input operating unit for allowing the user to input a driving condition; and a controller for controlling a torque of the first and second driving sources for driving at least one of said first and second joint axles of said robot arm, said controller exercising control so that a distal end of said robot arm exhibits elasticity, wherein said controller is operative, in response to said input operating unit receiving from the user an output direction of a distal end of the limb to be trained as the driving condition to use a first vector beginning at the first joint axle of the robot arm and terminating at the distal end of the robot arm to represent a first load direction and a second vector beginning at the second joint axle of the robot arm and terminating at the distal end of the robot arm to represent a second load direction to calculate elasticity of the robot arm in an output direction based on the output direction received from the user, the elasticity being different between the output direction received from the user and a direction substantially perpendicular to the output direction received from the user, wherein said controller is thereafter operative in response to displacements δ 1 and δ 2 of the joint angles from arbitrary articular axle torques τ u1 and τ u2 to calculate values of torques τ α1 and τ α2 by means of expression (1) below, the displacements being caused when the user applies force to the robot arm in the load direction and being measured by said joint angle sensors, wherein said controller controls said first and second driving sources so that the torques generated by said first and second driving sources are substantially equal to the values calculated, ( τ a ⁢ ⁢ 1 τ a ⁢ ⁢ 2 ) = ( τ μ ⁢ ⁢ 1 τ μ ⁢ ⁢ 2 ) - ( κ 11 κ 12 κ 21 κ 22 ) ⁢ ( ∂ 1 ∂ 2 ) ( 1 ) where κ 11 , κ 12 , κ 21 and κ 22 denote arbitrary elasticity parameters and where a training load input by the user and applied by the robot arm to the limb of the user is substantially equal to and opposed to the force applied by the user, the training load being opposite in direction to the output direction received from the user, wherein said controller sets, when the user inputs the output direction of the distal end of the limb to be trained and the training load, the training load input by the user as an upper limit of the training load generated by said robot arm, and sets zero as a lower limit of the training load generated by said robot arm, and wherein said robot arm is displaced when the user exerts a force exceeding the upper limit.