To provide a method for reducing the value of a content such as a waste or the like in a metal can be easily and safely evaporating a liquid component from the content.

The method for reducing the volume of content in a metal can comprises the steps of carrying the can 1 containing the content directly on the heater 2 having an induction heating coil, heating a metal can bottom plate by the coil to heat the content, simultaneously sucking and pressure reducing the can 1 by a sucking unit 30 coupled through a condenser 21, and evaporating the inner liquid component to reduce the volume of content. The can 1 for storing the content is used a sit is for the evaporating operation to eliminate the necessity of transferring the content to improve the operability. Thus, the heating and evaporating are conducted under a reduced pressure to lower the evaporating temperature and to prevent the burning damage of the coating of the can itself.