A connection relation detecting system according to the present invention includes a plurality of modules which include a plurality of connection ports which can connect with another module, and an information processing device which, when the plurality of modules are connected with each other, is connected to one of the plurality of modules through the connection ports. Each of the plurality of modules includes a connection changing unit which connects the connection ports inside the module in response to a connection change request from the information processing device. The information processing device includes a connection detecting unit which, when the information processing device makes the connection change request to the module, detects a module newly recognized through the module as a module connected to the module to which the connection change request is made.