Expansive demolition agents have been widely used to demolish rocks and concrete structures. To design the demolition works using expansive demolition agents rationally, it is important to estimate the expansive pressure exactly and to determine its action on the inner surface of a borehole. The authors have developed new pressure transducers which can directly measure the expansive pressure in a borehole. Several variables affecting the nature of expansive pressure in a borehole were investigated with the transducers. As a result, we determined the following: 1) Expansive pressure is transmitted in a manner similar to that of fluid. 2) The influence of the physical restraint of surrounding materials is very small. 3) The expansive pressure depends on thermal conditions including ambient temperature, but the relationship between expansive pressure and the degree of hydration of CaO is plotted on the same curve without dependence on temperature.