Provided is a novel carbonization treatment method for carbonizing a biomass material containing a large amount of water at an extremely low temperature, and a method for producing carbonized biomass. A water-containing biomass material is carbonized while maintaining the biomass material under treatment conditions including an oxygen-containing atmosphere and a temperature range of 70° C. or greater and less than 100° C., without a drying step for removing or reducing the water forcibly. At this time, preferably the water content (percentage) of the biomass material at the start of carbonization while maintained under the treatment conditions is within a range of 40 to 80% inclusive, and preferably the biomass material is thus maintained for two weeks or longer. Further, as the biomass material, one material or a mixture of two or more materials selected from waste biomass materials and plant (cultivated crop) biomass materials such as food waste, livestock excrement, agricultural waste, marine waste, and forest waste, can be applied.