To provide a charged pressure measuring instrument wherein a pressure change, time, a flow rate, and temperature are measured to calculate the charged pressure from the physical properties of a gas, thereby enhancing the measurement accuracy of the charged pressure.

This charged pressure measuring instrument 1 for measuring a charged pressure in a pressure vessel 21 such as an accumulator, has a means for pressurizing the interior of the pressure vessel by pressing a fluid into a fluid chamber 25 of the pressure vessel 21, a means for measuring a flow rate, time, pressure, and temperature during the pressurization by the pressurizing means, and a personal computer 8 for collecting and computing measurement data obtained by the measuring means.