To provide a parasitic antenna capable of expanding the communication range of a radio device installed in a metal shield body such as a pipe shaft.

A radio wave transmitted from the radio device 20 is received by an antenna 1 provided as a microstrip antenna nearby the radio device 20 and fed to conductor sheets 2 and 2 through a feeder 3. The conductor sheets 2 and 2 are stuck nearby the length center of a slit 11 of a conductor plate 10 across the slit 11, which then operates as a slot antenna to radiate the radio wave fed from the feeder 3, thereby operating as a parasitic antenna. The parasitic antenna is installed between a master station and a slave station of a meter-reading data gathering system wherein meter-reading data representing use states of lifelines of water supply, gas supply, etc., are gathered from radio slave stations through a radio master station and sent to a center.