Natural slopes consisting of weathered granite rocks (Masado) have frequently failed during heavy rainfalls. In Yamaguchi prefecture, it is necessary to determine the strength of Masado using slope stability analysis. In order to obtain the in-situ strength parameters of undisturbed Masados, field shear tests using a field shear test apparatus were carried out at 9 sites in Yamaguchi and Hiroshima prefectures. The effect of the side friction and maximum grain size of the soils on the test results was investigates. Furthermore, constant volume shear tests were performed to remove the effect of the side friction.