To provide a waste liquid treatment method capable of more effective utilization of wastes such as plastic waste than conventional methods.

A waste liquid treatment method includes a step for subjecting wastes such as plastic waste to heat decomposition by induction heating to obtain charcoal, a step for introducing waste liquid into a storage tank 2 of the charcoal for the charcoal to adsorb contaminants in the waste liquid, and a step for heating the charcoal by induction heating to subject the contaminants to heat decomposition, wherein the charcoal having the contaminants heat-decomposed is sent back to the storage tank 2. The charcoal is heated by induction heating (for example, heated up to 900°C or higher) to have the contaminants (such as organic components) heat-decomposed and the charcoal thus deprived of the contaminants is sent back to the storage tank. The charcoal is thus reactivated (recycled) and repeatedly used for cleaning waste liquid.