To provide a subgrade exploration device capable of exploring a cavity or the like present in the subgrade.

A subgrade exploration device X1 includes: an excavation shaft 1 for excavating a subgrade G; earth pressure detection means 2 having a pressure receiving member 10 receiving an earth pressure from the subgrade G backward when the excavation shaft 1 excavates in the subgrade G and moved backward by the earth pressure, and a pressing member 20 giving a forward pressing force to the pressure receiving member 10 and moving the pressure receiving member 10 forwardly by the pressing force according to reduction of the earth pressure, and provided at a tip 1A of the excavation shaft 1; and earth pressure calculation means for calculating the earth pressure by utilizing forward and backward movement of the pressure receiving member 10.