An industrially advantageous process for producing 3-chloromethyl-3-cephem derivative crystals. The process for 3-chloromethyl-3-cephem derivative production comprises: a first step in which a thiazolineazetidinone derivative (1) is reacted with a sulfonyl halide (2) in the presence of an acid in a solvent to obtain an azetidinone derivative (3); a second step in which the azetidinone derivative (3) is reacted with a chlorinating agent in an organic solvent to obtain a chlorinated azetidinone derivative (4); and a third step in which the chlorinated azetidinone derivative (4) is reacted with an alcoholate (5) at a pH of 8 or lower in a solvent comprising an alcohol and an ether and a 3-chloromethyl-3-cephem derivative (6) is recovered in the form of crystals.