1. A dynamo control device for a dynamometer system, the dynamometer system including a dynamometer connected to a test piece generating power 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, wherein the controller is designed by applying a control system design method called an H ∞ control or a μ synthesis to a generalized plant that outputs a prescribed observation output 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 so that responsiveness from the external input to the controlled variable decreases, 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, and 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 a test piece unit 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.