To reduce a radio transmission loss, to reduce transmission line installation cost and to realize secure and stable communication/call by setting sewerage to be a ratio transmission line, installing prescribed reflectors at the right angles branch points of the sewerage, thereby constituting the transmission line.

A reflector A: 8 reflecting in one direction and a reflector B: 9 reflecting in two direction are properly arranged in the branch points of a sewer 13. In the reflectors A: 8 and B: 9, vertical and lateral sizes are set to be the length of one wavelength of a transmission radio frequency. They are miniaturized and reflection is made efficient. They are disposed to an upper part in the sewer so that the sewage flow is not obstructed. A radio repeater 2 executes transmission/reception with the radio base station by an intra- sewerage transmission/reception antenna 10 through the reflectors A: 8 and B: 9. Thus, a radio transmission line loss can be reduced even in the sewerage having a right-angled part. The element of the intra-sewerage antenna 10 is constituted of an orthogonal polarized wave. Thus, fading is reduced and stable and secure data communication is realized.