Extension tests on Shirahama sandstone (blocks I and II) and Shikotsu welded tuff were carried out under confining pressure. Tensile strength increased and then decreased for Shirahama sandstone block I and Shikotsu welded tuff. It decreased or unaffected with confining pressure for Shirahama sandstone block II. Absolute value of lateral strain at peak load point increased with confining pressure and that in uniaxial compression tests distributed in the similar manner on the confining pressure-strain value plot. Absolute value of axial strain at peak load point increased with confining pressure except for Shirahama sandstone block I. 50% tangent Young's modulus increased with confining pressure and that in uniaxial compression tests distributed in a similar manner on the confining pressure-Young's modulus plot. Effect of confining pressure on Poisson's ratio was unclear. Residual strength decreased with confining pressure except for Shirahama sandstone block I. Results of Brazilian tests distribute upper limit of results in extension test under confining pressure on the confining pressure-tensile strength plot except Shirahama sandstone block II which distributed above the extension test results. The effect of confining pressure on tensile strength was roughly explained by assuming a number of discontinuities whose strength was represented by Coulomb criterion and whose angle was different with each other.