In nurse call systems where communication between a plurality of slave units and a plurality of extension terminals is possible, a nurse call controller serving the slave units and a private branch exchange serving the mobile terminals have been connected to each other through a nurse call adaptor; thus, there is a need for one such nurse call system that does not require a nurse call adaptor. Provided is an interface unit which is connected to a nurse call controller through a plurality of ISND basic interfaces, connected to a private branch exchange through an ISDN primary rate interface, and performs interface conversion therebetween.