Various testing methods for adhesive strength are prescribed in JIS but no testing method is indicated under no singular stress field. Usually, strength of materials should be considered for plane specimens where uniform stress distributions can be expected. However, in the case of adhesive strength evaluation, all bonded specimens exhibit a singular stress field, the intensity of which (ISSF) varies depending on the adhesive layer geometry. This is the reason why the adhesive strength varies depending on the adhesive layer geometry. In this paper, therefore, a protruding butt joint is proposed to obtain a constant interface stress distribution. Then, the essential adhesive strength is clarified under uniform interface stress without localized stress concentrations. Unlike standard butt joints, where strength is significantly affected by adhesive layer thickness h, it was found that the adhesive strength in the proposed joint remains constant at critical stress σ_B = 47.7 MPa, regardless of h. It is also found that in JIS butt joints, the border at h=0.1 mm allows a distinction between fracture due to internal stress and fracture due to a singular stress field. In the case of failure due to a singular stress field, by calculating the region (process zone) where the average stress is the original strength of the butt joint, σ_B = 47.7 MPa, it is thought that failure will occur at a position r_B = 14.7 μm from the adhesive end. This is almost equal to the fracture origin of a rectangular column butt joint reported in a previous study, and can be said to be a reliable evaluation method.