To provide a strain gage capable of measuring the self-contracting behavior of concrete just after the cement material is placed and useful for making clear the mechanism of a cracking phenomenon of the concrete caused by the self-contraction, for controlling cracking, and for performing design.

This strain gage 4 is formed as a thin-walled oblong parallelepiped molding 3 by air-tightly coating the whole of a strain gage element 1 and the proximity of a connection part between the gage element 1 and lead wires 2 with a low-elasticity silicone resin. The gage 4 is embedded in just-mixed concrete to be measured and the lead wires 2 are connected to a strain measuring instrument. The gage element 1 in the molding 3 embedded in the concrete varies in resistance value due to its self-contracting behavior while the just- placed concrete in a flowable state becomes hard. The resistance value is measured by the measuring instrument and converted into displacement.