An indoor master unit 200 divides image information stored in an image information storage unit 203 and taken by a camera unit 101 of a front door slave unit 100 into a length capable of being transmitted in one TDMA time slot, and writes a transmission sequence number, respectively, and transmits the same information several times. The plurality of wireless monitor slave units 300a to 300n at the receiver side delete an error packet and repetitively received image information, and perform image display.