An object of the present invention is to provide a module connecting system that can comply flexibly with the number of expansion modules and has an inexpensive and simple configuration. In the configuration of the module connecting system of the present invention, a basic module transmits a repetitive pulse signal to an expansion module, the repetitive pulse signal whose frequency is divided by a frequency dividing circuit included in the expansion module is output as frequency dividing information to the basic module, and the number of expansion modules is determined based on the frequency dividing information.