Soil mechanics study plays a key role in various engineering problems relating to future lunar/planetary surface explorations. In-situ measurement of regolith physical properties is of fundamental importance in developing a framework of lunar/planetary soil mechanics. An in-situ soil investigation package; LSM (lunar soil mechanics investigation system) is being developed as a moon utilization payload for Japan's Moon lander SELENE-2, which is planned as a follow-on program of Kaguya program (SELENE). This paper focuses on a soil testing tool, which is one of the subsystems of LSM, and describes a result of laboratory experiments using a prototype model. The equipment developed is a small-diameter tool with a paired-inflatable blade and is used in a pre-frilled borehole for performing two types of soil mechanics test; lateral loading test and direct shear test against the borehole sidewall. In this study, a series of shear tests were demonstrated with a Japanese lunar soil simulant (FJS-1) and the results were compared with results of a triaxial compression test and direct shear box test. The test results showed that the proposed method has a great potential to be a reliable tool that can provide strength parameters on the lunar surface. The characteristics and underlying limitations of the probe were also discussed for future improvements.