We made a pyramidal horn antenna system for 21cm cosmic radio wave. The antenna system has a rectangular waveguide with TE10 TE10 mode and a copper probe to detect the electromagnetic wave in waveguide. The parameters of the probe are obtained by experiments using two waveguides. Pyramidal horn antenna is designed to get a gain of 20dB. The size of the aperture of this horn is 94cm×74cm 94cm×74cm . The beam width of half power is about 14.7∘ 14.7∘ for the frequency of 1.4GHz. The horn is made of aluminum plate with a thickness of 2mm. The signal from horn and waveguide is amplified in a receiver system and is converted into DC in the detecter. The intensity of the signal is recorded on a chart recorder. We observed the sun using this simple radio telescope and obtained the brightness temperature of 2.2×105K 2.2×105K , which indicates that the sun is quiet at 21cm.