Recently, damage in the adhesive layer has been investigated in-situ microscopic observation under several load conditions. However, observations are limited on the surface of the adhesive layer. If the tensile test of the butt joint whose adherend thickness is approximately as same as the thickness of adhesive layer can be realized, damage in the interior of the adhesive layer may be observed using transmitted light. This will facilitate elucidation of the fracture mechanism of adheisvely bonded joints. In this study, adhesively bonded butt joint specimens were made using thin steel plate with thicknesses of 0.3 mm and 1 mm, where thiokol modified and rubber modified adhesives were used. Using these butt joints, two kinds of tensile tests were conducted ; One to obtain stress-strain curves of these butt joints, and the other to investigate the damage in the adhesive layer in-situ microscopic observation. Moreover, stress distributions in the adhesive layer were also analyzed using the finite element method. Main results obtained are as follows. (1) The stress-strain curves indicated that yielding behavior of the butt joints bonded by thiokol modified adhesive was mainly due to the plasticity of the adhesive layer, while that joint of joints using rubber modified adhesive was mainly due to damage of the adhesive layer. (2) In-situ observation indicated that for butt joints bonded with thiokol modified adhesive. initial debonding occurs at the adhesive/adherend interface near the end of the adhesive layer where maximum shear stress is concentrated. On the other hand, for butt joints bonded with rubber modified adhesive stress whitening was observed in the adhesive layer, where the whitening zone agreed well with the counter of the mean hydrostatic stress. Then, initial crack appeared in the middle of the adhesive layer where mean hydrostatic stress was higher than at the adhesive/adherend interface.