An adjusting device for a pressure detector that is capable of arbitrarily adjusting the initial position of a membrane member is provided. An adjusting device for a pressure detector includes a control unit that sequentially executes a first step in which a predetermined adjusting pressure is generated in a first zone by activating a pump with the first zone being closed by the closing of an electromagnetic valve, and a second step in which a gas-phase portion, the first zone, and a second zone are combined into a closed space by disabling the closing of the electromagnetic valve; and an adjusting-pressure-acquiring unit that acquires the adjusting pressure to be generated in the first step, the adjusting pressure being acquired in accordance with a relationship between a pressure and a capacity of the first zone in the first step and a pressure and a capacity of the combination of the gas-phase portion, the first zone, and the second zone in the second step. The control unit brings the membrane member to a position corresponding to the adjusting pressure by executing the second step after activating the pump in the first step in such a manner as to generate the adjusting pressure acquired by the adjusting-pressure-acquiring unit.