A shaft torque control device executes highly responsive shaft-torque control even when spring rigidity of a connection shaft connecting an engine and dynamometer varies, and has a feedback control system including a nominal plant imitating input-output characteristics of a test system, generalized plant having nominal plant; controller providing an input with use of outputs and variation term causing variation in the nominal plant on the basis of a variation transfer function. In the controller, setting is made to satisfy a design condition. Nominal plant is structured with a two-inertia system configured by connecting two inertia bodies via a shaft having spring rigidity equal to a predetermined nominal value set to be a lower limit value in an assumed variation range of spring rigidity of the connection shaft. The variation transfer function is a positive real function. Spring rigidity in the nominal plant Na increases from the nominal value.