The pressure distribution on the propeller blade is usually predicted in Japan by using the lifting surface theory and Sugai's concept of the equivalent two-dimensional wing. Though the method is admitted to be useful through comparison between experiment and calculation, the effectiveness of the concept is not seemed to be garanteed by the theoretical calculations. Then we, as the first step, show that the concept is accurate enough for practical usage by comparing the pressure distributions which are obtained by the thick wing theory and the above-described method applied for the three-dimensional wings with several plan forms and wing sections. Conclusions are as follows. 1) The concept of equivalent 2-D wing predicts fairly well the pressure distribution on the 3-D wing, which is obtained by the thick wing theory. 2) The pressure distribution on the equivalent 2-D wing has a tendency to deviate from the one on the 3-D wing in case of thick wing or small aspect ratio wing or the swept wing and in case the section approaches the wing tip. 3) The shape of wing section does not affect the agreement of pressure distributions obtained by two methods. 4) The equivalent camber and angle of attack change considerably along the span and are different from those of the actual 3-D wing.