A hollow-fiber membrane module comprising a hollow-fiber membrane sheet formed by arranging a hollow-fiber membrane in a sheet shape or a stacked body thereof and a case-shaped water collecting member holding the hollow-fiber membrane sheet or the stacked body thereof, the water collecting member having an elongated box shape, wherein one surface of the water collecting member has an opening part for fixing the hollow-fiber membrane sheet stacked body therein, the inside of the opening part being filled with a fixing resin for fixing the hollow-fiber membrane sheet stacked body to the water collecting member, and the hollow-fiber membrane sheet stacked body and the water collecting member being liquid tightly sealed by this fixing resin, wherein the water collecting member has a length, width and height dimension, wherein the water collecting member has a thickness, corresponding to a width, of 20 mm or less along an out-of-plane direction of the hollow-fiber membrane sheet, a basic width of the water collecting member is 15 mm or less, and a thickness of the side wall of the water collecting member extending along the opening for fixation in the out-of-plane direction of the hollow-fiber membrane sheet is 2 mm or less, wherein the water collecting member has a water collecting path for collecting treated water filtered through a hollow-fiber membrane, and a water intake port, the water intake port being in an elliptical shape having a minor axis extending in the out-of-plane direction of the hollow-fiber membrane sheet, the water intake port being formed on at least one end face of the water collecting member, and wherein a thick wall part having a thickened thickness which is thicker than the basic width in the out-of-plane direction of the hollow-fiber membrane sheet of the water collecting member is equipped on the at least one end face of the water collecting member having the water intake port formed thereon, and a proportion of a total area of a cross-section of the hollow-fiber membrane in a horizontal cross-section of the opening part for fixation at an arbitrary position is at least 45% and the upper limit of the proportion is 90% or less, and is expressed by Equation: n ⁢ π ⁢ r 2 / WL wherein n represents the number of hollow-fiber membrane when the width of the opening part for fixation, when viewed from the upper surface, is denoted as W, the length is denoted as L, and the radius of the hollow-fiber membrane is denoted as r. wherein one surface of the water collecting member has an opening part for fixing the hollow-fiber membrane sheet stacked body therein, the inside of the opening part being filled with a fixing resin for fixing the hollow-fiber membrane sheet stacked body to the water collecting member, and the hollow-fiber membrane sheet stacked body and the water collecting member being liquid tightly sealed by this fixing resin, wherein the water collecting member has a length, width and height dimension, wherein the water collecting member has a thickness, corresponding to a width, of 20 mm or less along an out-of-plane direction of the hollow-fiber membrane sheet, a basic width of the water collecting member is 15 mm or less, and a thickness of the side wall of the water collecting member extending along the opening for fixation in the out-of-plane direction of the hollow-fiber membrane sheet is 2 mm or less, wherein the water collecting member has a water collecting path for collecting treated water filtered through a hollow-fiber membrane, and a water intake port, the water intake port being in an elliptical shape having a minor axis extending in the out-of-plane direction of the hollow-fiber membrane sheet, the water intake port being formed on at least one end face of the water collecting member, and wherein a thick wall part having a thickened thickness which is thicker than the basic width in the out-of-plane direction of the hollow-fiber membrane sheet of the water collecting member is equipped on the at least one end face of the water collecting member having the water intake port formed thereon, and a proportion of a total area of a cross-section of the hollow-fiber membrane in a horizontal cross-section of the opening part for fixation at an arbitrary position is at least 45% and the upper limit of the proportion is 90% or less, and is expressed by Equation: n ⁢ π ⁢ r 2 / WL wherein n represents the number of hollow-fiber membrane when the width of the opening part for fixation, when viewed from the upper surface, is denoted as W, the length is denoted as L, and the radius of the hollow-fiber membrane is denoted as r.