A low cross-polarization slot array antenna in the narrow wall of ridge waveguide is presented. A non-angled slot which is created in the narrow wall of the ridge waveguide is considered to be used as radiating resonant slot. The normalized resistance and normalized reactance curves are presented for design purposes. A six element slot array waveguide antenna with ridges on its narrow wall is then designed at frequency of 9 GHz using the normalized conductance of each radiating slot. It is shown that good performance can be achieved even for angled ridge in waveguide. The antenna has the characteristics SLL of -20 dB and cross-polarization equivalent to -60 dB.