In this study, a consideration was conducted regarding a method for calculating ultimate shear strength of reinforced concrete beams by means of a plastic analysis of truss and arch mechanisms in the design guidelines for earthquake resistant concrete buildings based on ultimate strength concept, and an improved method with good accuracy in average was suggested. The values of effective factor, ν of concrete compressive strength were inversely analyzed based on the arch mechanism from the past experimental results on ultimate shear strength of reinforced concrete beams without shear reinforcement, and an equation (17) was induced based on a consideration on compressive strength of concrete partitioned into strips by cracking. As for an assumption of cot φ in the truss mechanism, it was shown that the method A in the design guidelines gives better agreement with the experimental results than the method B does.