The present invention aims to provide a shrinkage reducing agent that exhibits high performance to reduce the shrinkage of hydraulic materials. The present invention relates to a shrinkage reducing agent for a hydraulic material including a compound that satisfies the following conditions (1) to (4): (1) a ratio represented by {(15-drop flow value of a mortar composition containing the compound)/(15-drop flow value of a mortar free from the compound)}×100 is 120 or less, wherein the (15-drop flow value of a mortar composition containing the compound) means a 15-drop flow value of a mortar composition containing the compound at a solid concentration of 0.1% after 10 minutes mixing and the (15-drop flow value of a mortar free from the compound) means a 15-drop flow value of a mortar free from the compound after 10 minutes mixing; (2) a ratio represented by {(ratio between 15-drop flow values after two hours mixing)/(ratio between 15-drop flow values after 10 minutes mixing)}×100 is 110 or less, wherein the (ratio between 15-drop flow values after two hours mixing) means a ratio of a 15-drop flow value of a mortar composition containing the compound at a solid concentration of 0.1% after two hours mixing to a 15-drop flow value of a mortar free from the compound after two hours mixing, and the (ratio between 15-drop flow values after 10 minutes mixing) means a ratio of a 15-drop flow value of a mortar composition containing the compound at a solid concentration of 0.1% after 10 minutes mixing to a 15-drop flow value of a mortar free from the compound after 10 minutes mixing; (3) the compound has an average particle diameter of 2.2 to 8.5 nm in an alkaline solution; and (4) a product of {(ratio between 15-drop flow values relating to a mortar composition containing the compound at a solid concentration of 1.0%)×(ratio between 15-drop flow values relating to a mortar composition containing the compound at a solid concentration of 0.1%)} is 10,100 or more, wherein the (ratio between 15-drop flow values relating to a mortar composition containing the compound at a solid concentration of 1.0%) means a ratio of a 15-drop flow value of a mortar composition containing the compound at a solid concentration of 1.0% after 10 minutes mixing to a 15-drop flow value of a mortar free from the compound after 10 minutes mixing, and the (ratio between 15-drop flow values relating to a mortar composition containing the compound at a solid concentration of 0.1%) means a ratio of a 15-drop flow value of a mortar composition containing the compound at a solid concentration of 0.1% after 10 minutes mixing to a 15-drop flow value of a mortar free from the compound after 10 minutes mixing, the mortar compositions and the mortars are in accordance with JASS 5 M402.