A demolition agent which acts by means of expansive stress generated upon hydration and which is used as a slurry filled into holes of an object to be demolished is obtained by mixing a calcareous material, a siliceous material and gypsum in the molar ratio for CaO/Sio 2 of 5.0 - 14.4 and for CaSO 4 /SiO 2 = 0.13 - 1.19, roasting the mixture under an oxidizing atmosphere to 1350 -1500°C and comminuting the calcined material. The main components of the matter thus obtained are 24 - 65% by weight of 3CaO.SiO 2 crystals, 30 to 60% by weight of CaO crystalline particles and 5 - 17% by weight of CaSO 4 , more than 50% by weight of the CaO crystalline particles being included in the 3CaO.Si0 2 crystals.