Surface thermal lens (STL) signal produced by a cw modulated top-hat beam excitation is detected by an obliquely-incident probe beam. The dependence of the STL amplitude on the incident angle theta of the probe beam is experimentally investigated. The STL amplitude as a function of the detection distance is measured at different angles of incidence. The results show that the maximum STL amplitude and the optimum detection distance, where the maximum STL amplitude is obtained, decrease by cos theta as the incident angle increases. These experimental dependences are well explained by a simple theoretical model.