The present invention relates to an apparatus for measuring pressure inside a predetermined vessel based on the principle that the transmitting efficiency of ultrasonic waves is changed by acoustic impedance variation according to an internal pressure. The apparatus includes an ultrasound exciting unit 20 disposed inside the vessel 10 and generating predetermined ultrasonic waves, an ultrasound receiving unit 30 disposed inside the vessel 10 and placed on the same axis line as that of the ultrasound exciting unit 20, a control unit 70 for controlling a frequency and a waveform of the excitation signal transmitted into the ultrasound exciting unit 20, and a pressure measuring unit 80 for measuring an internal pressure of the vessel 10 based on an ultrasonic signal received by the ultrasound receiving unit 30 and the excitation signal transmitted into the ultrasound exciting unit 20.