To produce an alkali silicate-based inorganic hardened body having high mechanical strength, excellent water resistance, moisture absorbing and releasing function by a simple production method and to provide a method for producing the hardened body.

The inorganic hardened body is obtained by heating and hardening a mixture of silica raw material and sodium hydroxide as main components having a weight ratio of the silica content in the silica raw material to sodium content in the sodium hydroxide of 0.05≤Na2O/SiO2≤0.20 in the presence of water. The inorganic hardened body preferably has ≥1 MPa wet flexural strength and ≥100 g/cm2 moisture absorbing and releasing amount. The method for producing the inorganic hardened body comprises mixing the silica raw material with sodium hydroxide so as to make the range, kneading and stirring the mixture in the presence of water, forming the mixture, heating and hardening the formed material in a dryer at 80-250°C or spraying the mixture into a layer and heating and hardening the layer by a hot press at 100-250°C.