A method and apparatus for treating wastes by two-stage gasification recovers metals or ash content in the wastes in such a state that they can be recycled, and gases containing carbon monoxide (CO) and hydrogen gas (H2) for use as synthesis gas for ammonia (NH3) or production of hydrogen gas. The wastes are gasified in a fluidized-bed reactor at a low temperature. Then, gaseous material and char produced in the fluidized-bed reactor are introduced into a high-temperature combustor, and gasified at a high temperature and ash content is converted into molten slag. After water scrubbing and a CO conversion reaction, the gas is separated into H2 and residual gas. The residual gas is then supplied to the fluidized-bed reactor as a fluidizing gas.