A method for sterilizing microbial cells is provided. According to the method, microbial cells or a culture containing microbial cells are treated with a polyethylene glycol-based nonionic surfactant so that almost all of the microbial cells are sterilized while the enzyme activity expressed in the microbial cells is maintained at a high level. A method for sterilizing microbial cells and a material containing the sterilized microbial cells, in which the microbial cells are sterilized using a polyethylene glycol-based nonionic surfactant, can be used for foods so that the microbial cells are sterilized to be used in food production. Further, a material containing sterilized microbial cells can be used in processes for preparing tagatose, in which Corynebacterium genus microbial cells that produce Galactose and/or Arabinose isomerase are sterilized using a polyethylene glycol-based nonionic surfactant.