Disclosed herein are an apparatus and method for gas leakage measurement in a high pressure reactor. In particular, the present invention relates to an apparatus for gas leakage measurement in a high pressure reactor including: a gas inlet; a first valve equipped to one side of the gas inlet; a flowmeter; a pressure gauge measuring an internal pressure of the reactor; a gas outlet; and a second valve, wherein if in internal pressure reaches a preset pressure by closing the second valve and injecting a gas to the reactor through the flowmeter in the state of opening the first valve, the gas leakage status in the reactor is measured through pressure changes in the pressure gauge after closing the first valve.