A relay device and a pressure detection device are provided that are configured to eliminate the necessity of calibrating an output value from a pressure sensor in the pressure detection device even when a mold is replaced. A relay device for a pressure detection device provided with a pressure sensor (S1 to Sn) configured to detect an inner pressure in a cavity (CT) of a mold of an injection molding machine is provided. The relay device is integrally fixed to the mold. The relay device includes a storage configured to store specific information of the pressure sensor (S1 to Sn) provided to the mold to detect the inner pressure of the cavity (CT), the cavity (CT) being defined between a fixed-side mold and a movable-side mold of the mold.