PURPOSE: To obtain a security system using a cordless telephone set detecting the invader from the outside by plural slave machines.

CONSTITUTION: Infrared ray sensors 9, 18 and 19 are incorporated in the slave machines A8, B16 and C17 of a cordless telephone set, and if either one of the infrared ray sensors 9, 18 and 19 of the slave machines A8, B16 and C17 detects an invader in a burglar prevention set mode, a burglar preventing correspondence such as the simultaneous sounding of alarm sound, etc., is performed by the slave machines A8, B16 and C17 and a master machine 24. By this constitution, various kinds of large-scaled installation instructions for the burglar prevention are unnecessitated, a burglar prevention system which can be easily installed at a place where an invasion detection is required can be constituted, and the system becomes inexpensive because a cordless telephone set which is used as a telephone set is utilized.