A device for recovering heat of molten slag, having a simple structure and high heat recovery efficiency. The device includes: a fluidized bed (3) formed of a fluidized bed material (32) composed of a crushed solidified slag; a slag pool (4) for feeding a molten slag to the fluidized bed; heat transfer tubes (7) for recovering heat from the fluidized bed; and a solidified slag withdrawing device (21) for recovering a solidified slag (34), such solidified slag (34) being formed in the fluidized bed as the molten slag is solidified therein. Molten slag droplets (31) are dripped into the fluidized bed (3) from the slag pool (4) and solidified in the corresponding fluidized bed. The heat released at that time is transferred to the heat transfer tubes (7) through the bed material (32). Further, heat-transfer coefficient between the heat transfer tubes and the bed material is about 10 times larger than that between the heat transfer tubes and gas, thereby reducing heat-transfer area for required heat recovery amount, thus making it possible to reduce the size of the heat recovery device.