To provide a method of treating molten slag which has excellent productivity and effective profitability and by which the production of gaseous hydrogen in the mortar or the concrete by the reaction of metal aluminum in the molten slag with calcium hydroxide and water is prevented even when the molten slag is used as aggregate for the mortar or the concrete and the molten slag is effectively used to manufacture the mortar or the concrete having uniform quality and to provide the mortar or the concrete using the molten slag obtained by the method as the aggregate.

The method of treating the molten slag is carried out by dipping the molten slag in ready-mixed concrete sludge water to reaction-treat metal aluminum contained in the molten slag to manufacture the aggregate for the mortar or the concrete. As the ready-mixed concrete sludge water, ready-mixed concrete sludge water produced in a ready-mixed concrete plant or a concrete secondary product plant is used.