The present invention discloses a submerged hollow fiber membrane module which is of such a structure that it is easy to expand a module processing capability according to a treatment capacity, provides convenient module coupling properties and module manufacturing properties, maintains a stable flux under an efficient air diffusion condition and prevents the damage of membranes and water leakage caused by the loosening of module connecting regions. The submerged hollow fiber membrane module comprises [I] two module headers (2 and 2') having a filtrate water collecting portion (3) for collecting filtrate water filtered through hollow fiber membranes and a filtrate water outlet (7), [II] an air diffusion unit 8 consisting of support tubes (9 and 9') fixing the two module headers (2 and 2') while keeping them spaced a predetermined distance and air diffusion tubes (11 and 11') having air diffusion holes (13), and [III] a bundle of hollow fiber membranes (1) having both opposite ends fixed to the insides of the module headers (2 and 2') by an adhesive (6) so as to form a water collecting space within the module headers (2 and 2'), the ends (5) of the hollow portions of the hollow fiber (20) membranes being opened and disposed in parallel to a filtrate water discharge surface (4).