One electrical insulation evaluation technology is the pulsed electroacoustic method, which detects acoustic pressure due to the induced charge at the interface between the film and the electrode, and the space charge in the bulk. To improve its reliability, a measurement system for the apparent acoustic impedance of the film is introduced into the space charge measurement system. The apparent acoustic impedance of a polyethylene terephthalate (PET) film increased as the applied mechanical pressure increased. The position of the signal around the earth (lower) electrode shifted under a low mechanical pressure of 21.21 kN/m2, and the signal peak intensity around the earth electrode decreased at 600 s after the voltage application. On the other hand, the peak position shift and the decrease in peak intensity on the signal around the earth electrode were not observed under the high mechanical pressure of 1707 kN/m2. In the case of low mechanical pressure application, a large silicone oil layer between the PET and the earth electrode led to a charge shift due to interfacial polarization and a small peak intensity of the pressure wave generated around the interface.