To determine simply, rapidly, surely and accurately whether a unit water quantity of a concrete to be cast is proper or not for the concrete placed in a structure.

In the concrete 2 cast in the structure, a simple sensing device 1 is installed, and inserted at a slow speed. Whether the unit water quantity of the cast concrete is proper or not is determined by the presence or absence of a measured value of an electrostatic capacity/impedance of a measuring apparatus 4 when a display 3 is lit within a range of a reference value of reference blending and a management value of management blending, and only proper concrete is cast.