1. A dynamo control device for a dynamometer system, the dynamometer system including a dynamometer connected to a test piece generating torque in response to a command from an external control device through an intermediate connection body, a torque detector detecting torsional torque of the intermediate connection body, a rotation speed detector detecting a rotation speed of the dynamometer, and an inverter supplying electric power to the dynamometer, the dynamo control device for the dynamometer system generating a torque current command to the inverter based on detection signals of the torque detector and the rotation speed detector so as to realize a no-load state where a load is not applied to the test piece, wherein the dynamo control device includes a controller designed by applying a control system design method called an H∞ control or a μ synthesis to a generalized plant, wherein the generalized plant outputs a first observation output corresponding to the detection signal of the torque detector, a second observation output corresponding to the detection signal of the rotation speed detector and a prescribed controlled variable from an external input corresponding to torque generated in the test piece and a control input corresponding to the torque current command, wherein the generalized plant includes a dynamic characteristic model identifying a characteristic of the dynamometer system so that angular acceleration of the test piece is output from the external input and the control input based on the equation of motion of a three-inertia system, wherein the three-inertia system is configured by connecting the test piece, the intermediate connection body and the dynamometer each having an original inertia moment, wherein the controlled variable of the generalized plant is a signal obtained by multiplying a weighting function having an integration characteristic by a difference between angular acceleration of the test piece in the state of being disconnected from the intermediate connection body and the dynamometer calculated by multiplying an inverse number of the inertia moment of the test piece by the external input and angular acceleration of the test piece calculated by the dynamic characteristic model, and wherein the controller is designed by numerical calculation based on the control system design method so that responsiveness from the external input to the controlled variable in the generalized plant decreases.