Two types of apparatus to treat steel slags with highly pressurized hot water were fabricated and examined in terms of leaching of constituents from the slags. The batch-type apparatus was consisted with a stainless steel vessel and a sand bath; a flow-type apparatus was composed of a stainless steel column, a column oven, an HPLC pump, a backpressure valve, and a fraction collector. In the batch-treating system, the concentrations of the elements leached from slags tended to increase as increase in the treating temperature while pH of the solution shows no obvious relations to the solution temperature. In the case of flow-treating system, on the other hand, a unique dependency of pH of the leachate on the treating temperature was observed in the obtained flow profiles. The flow profiles of the pH highly related to those of the concentration of calcium ion and sulfate. Treating 5.1 g of blast-furnace slag with 350 mL of water by the flow system at 250 ºC enabled to leach 22.5% of phosphorus, 97.1% of sulfur, and 0.89% of calcium from the slag. New depositions without sulfur on the treated slags were observed in SEM images, indicating that not only dissolution but also deposition occurred simultaneously during the treatment with highly pressurized hot water in the column. Thus, using highly pressurized hot water is promising method to treat steel slags for reuse of steel slags as environmental-friendly materials.