In order to measure the pressure under an apex seal and the related temperatures, the Mazda rotary engine was modified to operate as an overhanging eccentric shaft-type single-rotor research rig. This rig permitted the exposed center of the rotor to drive a multichannel packaged slip ring through a helical spring absorbing the eccentricity of the rotor. The pressure was measured with an inductance-type diaphragm pressure sensor by compensating the change in the gain due to the heat of combustion by the use of the measured sensor temperature. The effect of the seal configuration on the pressure has been analyzed by comparing the measured pressures with apex seals of 3mm and 6mm thicknesses with the theoretically calculated pressures, treating the vicinity of the apex seal as a labyrinth passage.