A method for determining the optimum ideal MHD $eta$limit and the operation conditions is presented for an axisymmetric tokamak reactor with a circular cross section. The $eta$limit is calculated under the constraints of ideal MHD instability criteria with varying the operation conditions which depend on the toroidal current density distributions. Semiempirical laws deduced from experimental observations are used for the toroidal current density distributions. Analytic derivations of various equations required to determine the $eta$limit are carried out from the empirical equations. Various distributions of the $eta$limit are obtained by the numerical calculations for different distributions of the toroidal current density. The resulting values of the maximum $eta$limited by ideal MHD instabilities are expressed by a scaling law in terms of the tokamak geometry and the safety factor.