An expansion material composition for cement mortar is provided to prevent cracking effectively, to correspond effectively to dry shrinkage and excessive firing shrinkage and to prevent degradation of a compressive strength. An expansion material composition for cement mortar comprises anhydrous gypsum 20~65 weight% of which particle-size is 150~200 mesh and which has SO3 of 48% or more, quicklime 30~75 weight% of which a particle-size is 100~175 mesh and which contains CaO of 80% or more, a calcium sulfur aluminate type mineral 1~20 weight% of which a particle-size is 150~200 mesh, calcium hydrooxide 1~20 weight% of which particle-size range is 150~200 mesh and which CaO of 65% or more and MgO of 5.0% or less. The calcium sulfur aluminate type mineral consists of CaO 65~75%, Al2O3 6.5~7.5% and SO3 18.0~19.0%.