This invention provides an inexpensive hollow-fiber membrane module that can prevent diaphragms from peeling or coming off from the cylindrical casing. The invention includes a hollow-fiber membrane module, comprising a cylindrical casing having an inlet port for the treating liquid at one end and an outlet port for the treating liquid at the other end, a hollow-fiber membrane bundle contained in the cylindrical casing, potting layers for fixing the hollow-fiber membrane bundle to the inner wall of the cylindrical casing at both the ends of the hollow-fiber membrane bundle, a header having an outlet port for the liquid undergoing treatment provided at one end of the cylindrical casing, and a header having an inlet port for the liquid undergoing treatment provided at the other end of the cylindrical casing, wherein baffle cylinders having slits extending in the direction at an angle to the axis of the cylindrical casing are fitted in the potting layers.