A separation membrane element comprising at least a water collecting pipe , separation membranes , a supply-side channel member , and a permeation-side channel member , wherein: the supply-side channel member is interposed between two surfaces of the separation membranes to form a supply-side channel; the supply-side channel member has a net shape in which plural fibrous rows X comprising fibrous objects A and arranged in one direction and plural fibrous rows Y comprising fibrous objects B and arranged in a different direction than the fibrous rows X cross each other sterically to form intersections (P 1 ); the fibrous objects A and the fibrous objects B have a large diameter portion and a small diameter portion along a longitudinal direction and are both made of a tapered fiber; in a longitudinal cross section including an arbitrary fibrous row and taken along a longitudinal direction of the arbitrary fibrous row, at least one of the fibrous objects A and the fibrous objects B comprise a thread that is thinner at a central portion located between intersection portions of the fibrous rows X and the fibrous rows Y than at the large diameter portion; a supply-side channel area ratio is in a range of 45% to 65%, wherein the supply-side channel area ratio in % is given by A 1 /A 2 × 100 where A 1 is the average area of the entire space within the thickness of intersection portions in a longitudinal cross section taken along longitudinal direction of a fibrous row and A 2 is the average area within the thickness of intersection portions between two adjacent intersection portions in the longitudinal cross section, and is determined as described in the description; and when a surface of the supply-side channel member is observed from a thickness direction, fibers between an arbitrary intersection and an adjacent intersection are tapered fibers whose diameter increase like a taper in a direction from one intersection to the other intersection. the supply-side channel member is interposed between two surfaces of the separation membranes to form a supply-side channel; the supply-side channel member has a net shape in which plural fibrous rows X comprising fibrous objects A and arranged in one direction and plural fibrous rows Y comprising fibrous objects B and arranged in a different direction than the fibrous rows X cross each other sterically to form intersections (P 1 ); the fibrous objects A and the fibrous objects B have a large diameter portion and a small diameter portion along a longitudinal direction and are both made of a tapered fiber; in a longitudinal cross section including an arbitrary fibrous row and taken along a longitudinal direction of the arbitrary fibrous row, at least one of the fibrous objects A and the fibrous objects B comprise a thread that is thinner at a central portion located between intersection portions of the fibrous rows X and the fibrous rows Y than at the large diameter portion; a supply-side channel area ratio is in a range of 45% to 65%, wherein the supply-side channel area ratio in % is given by A 1 /A 2 × 100 where A 1 is the average area of the entire space within the thickness of intersection portions in a longitudinal cross section taken along longitudinal direction of a fibrous row and A 2 is the average area within the thickness of intersection portions between two adjacent intersection portions in the longitudinal cross section, and is determined as described in the description; and when a surface of the supply-side channel member is observed from a thickness direction, fibers between an arbitrary intersection and an adjacent intersection are tapered fibers whose diameter increase like a taper in a direction from one intersection to the other intersection. <EOS>