PROBLEM TO BE SOLVED: To provide a method for treating a waste mushroom medium by which the waste mushroom medium can be reused at a low cost.SOLUTION: The method for treating the waste mushroom medium includes: a step of dehydrating the waste medium produced after cultivating the mushroom to afford a dehydrated waste medium and a squeezed liquid； a step of obtaining a culture solution by heat-sterilizing the squeezed liquid； a step of culturing yeast by inoculating the yeast in the culture solution； a step of separating fungus bodies of the yeast floated by proliferation from the culture solution； a step of cleaning the dehydrated waste medium； a step of dehydrating the cleaned waste medium and separating the cleaned waste medium into a second squeezed liquid and a dehydrated cleaned waste medium subjected to the dehydration treatment； and a step of obtaining a combustion base material of a recycled solid fuel by regulating the water content of the dehydrated cleaned waste medium.
