The equilibrium of the current-carrying toroidal plasma in the Heliotron C field is studied experimentally. The equilibrium state is very sensitive to a change in the transverse field of order 10 -3 of the main confining field. The effect of vertical magnetic field on the equilibrium is precisely examined by measuring the electrostatic equipotential surface of the plasma and the apparent electric conductivity of the plasma current. The observed values of the optimum vertical field B v , which is necessary for the equilibrium of a current-carrying toroidal plasma, are about 1/2∼1/3 of the theoretical values.