The present invention relates to a fuel cell membrane humidifier capable of preventing a humidification efficiency decrease caused by the pressure difference between the inside and the outside of the membrane humidifier. The fuel cell membrane humidifier according to an embodiment of the present invention comprises: a middle case in which a module insertion part, including an outer partition wall formed spaced apart from an inner wall of the middle case, is formed; a cap case coupled to the middle case; a hollow fiber membrane module inserted into the module insertion part; and an active pressure buffer part that is formed between the middle case and the module insertion part and prevents the module insertion part from expanding due to the pressure difference between the inside and the outside of the middle case, or resolves the pressure difference, according to the output status of the fuel cell.