Provided is a process by which an objective cephalosporin derivative having a high Z isomer content or an alkali metal salt thereof can be prepared via simple steps with industrial advantages. A process for the preparation of a cephalosporin derivative (4a) or an alkali metal salt thereof characterized by comprising the first step of bringing an aqueous solution of 7 amino 3 [(E/Z) 2 (4 methylthiazol 5 yl)vinyl] 3 cephem 4 carboxylic acid (1) or an alkali salt thereof into contact with an active carbon that has an iodine adsorptivity of 1200mg/g or more as determined by JIS K 1474 and a methylene blue adsorptivity of 250ml/g or more as determined thereby to prepare the carboxylic acid (1) having an enhanced content of Z isomer (2) or an alkali metal salt thereof and the second step of subjecting the carboxylic acid (1) which has been prepared in the first step and has an enhanced content of Z isomer (2) to reaction with a compound (3).
