A shaft torque control device includes a feedback control system having: a generalized plant P including a nominal plant model N representing the input/output characteristic of a test system including a tandem dynamometer system; and a controller K for applying a first and second control inputs u1, u2 to the generalized plant P on the basis of a first observed output y1 and second observed output y2. The controller K has been designed using a computer so as to satisfy a prescribed design condition. For the generalized plant P, defined are: a first control amount output z1 resulting from weighting the first observed output y1 using a weighting function Ge(s); a second control amount output z2 resulting from weighting a front transmission torque t1 using a weighting function Gt1(s); and a third control amount output z3 resulting from weighting a rear transmission torque t2 using a weighting function Gt2(s).