A cement setting accelerator comprising calcium hydroxide particles having an average particle diameter of 3 .mu.m or less, and a cement composition comprising 100 parts by weight of a cement and 0.05 to 10 parts by weight of the above setting accelerator. A wet ground product of calcium hydroxide or a fine precipitate formed by a reaction between a calcium salt and an alkali hydroxide can be used as the above calcium hydroxide particles.This setting accelerator shows a high setting accelerating effect and does not exert a bad influence upon the quality, especially durability of a hardened product of the cement composition.