In order to clarify the mechanism of solar radiation transfer in an urban surface layer, hemispherical reflectances of urban canyons for solar irradiation were investigated by field measurements using models made of concrete blocks and also by numerical calculations in consideration of the multiple reflections in the canyons. The urban canyon is formed by two opposite buildings of the same height. A compact designed spectrometer with an integrating sphere was used for the measurements of hemispherical reflectances. The dependence of the hemispherical reflectances on the canyon shape were made clear in connection with the solar elevation angle and the reflectances of the constituent surfaces. Further, the theoretical works using Monte Carlo method were carried out for the evaluation of the hemispherical reflection characteristics of urban surface layers composed of rectangular columns of different heights. Discussion was made on the effects of the height distribution and three-dimensional arrangement on the characteristics.