A method of preparing a mortar composition, comprising mixing of mortar materials comprising at least 25 to 45 parts by weight of high-early-strength portland cement, 40 to 60 parts by weight of sand, 0.05 to 0.12 parts by weight of polycarboxylic acid-based powdered water reducing agent, and 10 to 25 parts by weight of water, and not comprising any organic adhesives. Mixing of the mortar materials is achieved by agitating the same in an agitator until agitating load rapidly decreases. The mortar composition prepared in accordance with this invention has excellent ability to infiltrate into the surface to be adhered, and high bonding strength after hardening.