In order to examine the crossover from the Ising behavior to the renormalized Ising behavior, we have measured the coexistence curves of the ternary system, polystyrene I+ polystyrene II+ methylcyclohexane, by varying the molecular weight ratio r of polystyrene II to polystyrene I and volume fraction xi(2) of polystyrene II in total polystyrene. The observed critical exponent beta(t) for the concentration difference between two coexisting phases varies from 0.33 to 0.38 with the increase of parameter k=xi(2)r(r-1)(2). This result is consistent with the theory of Broseta and Leibler [J. Chem. Phys. 90, 6652 (1989)]. The exponent for the diameter mu(t) was found in the range from 0.81 to 0.92. The average of mu(t)=0.86 is close to the exponent mu for binary systems.