Provided is a shrinkage-reducing agent for a hydraulic material, which does not require a combination with any other admixture, is inexpensive, and is capable of suppressing a reduction in strength of a hardened concrete material, suppressing generation of a crack in a hardened concrete material by virtue of its excellent shrinkage-reducing ability, and imparting excellent freeze-thaw resistance. The shrinkage-reducing agent for a hydraulic material includes at least one kind of polyoxyalkylene compound (A) as an essential component, in which a concrete or a mortar produced by using the shrinkage-reducing agent for a hydraulic material has a spacing factor of 350 μm or less.