Abstract: A signal processing device 6 is connected to an endoscope 2 that examines a subject and outputs a signal in accordance with a result of the examination. The signal processing device 6 includes a plurality of internal modules 61 to 64 that process the signal output from the endoscope 2. The internal modules 61 to 64 are connected via an interface supporting a communication protocol and having a connector geometry, at least the communication protocol or both communication protocol and connector geometry conforming to a communication interface standard.