In order to check compliance for MHz-band wireless power transfer systems, we have proposed a calibration method for electric-field probe immersed inside a liquid phantom simulating a biological tissue by using a shortened dipole antenna. We use the proposed method to determine the distance characteristics of transmitting antenna factor of the shortened dipole antenna by using two-antenna method via the power transmission formula and, then, estimate the distance characteristics of average electric-field strength created by the shortened dipole antenna in the liquid phantom. In this paper, we construct a calibration system for an opto-electric probe immersed inside the liquid phantom with a concentration of 0.074 mol/l and calculate the conversion factor for the probe at 100 kHz and 6.8 MHz.