The method of measuring heat flow Q as a function of the sheath potential V is investigated in order to improve the time response of the thermal probe method. We found that the time constant τ for reaching a steady state in thermal phenomena was 123 ± 18 s. The result of τ indicated that the applicable method for measuring the Q-V characteristic was biasing the thermal probe in a stepwise manner, and this was verified experimentally. Although there existed a V-dependent offset of the Q-V curve deduced from the temperature gradients against that calculated from the ion current, a procedure to deduce the ion temperature T_i is developed. The experimental error in T_i was too large, and was caused by the error in Q. Thus we have to improve the accuracy of the Q measurement.