A signal processing device includes: a superposing circuit for generating a superposed signal obtained by superposing a data signal indicative of prescribed information on an operation signal relating to an operation element of an electric apparatus; an information storage part for storing the prescribed information; a signal transmission terminal for inputting/outputting the operation signal or the superposed signal to/from the electric apparatus or apparatus control devices; and an information transmission terminal for inputting/outputting information to/from an information transfer device which executes reading of information stored in the information storage part and/or writing of information in the information storage part. The signal processing device is connected to an external device via a connector member. Each of the terminals is provided in an area that is inside the connector member in a state where the signal processing device is connected to the external device via the connector member.