A dialysis catheter (1; 61) comprising: a catheter main body having a first lumen (16b; 66c) which communicates with a tip-end hole (22; 62) defining a tip-end opening that opens at a tip end of the catheter main body, and a second lumen (16a; 66b) which communicates with a side hole (23; 63, 64) that opens in a region of the tip end of the catheter main body, wherein: the side hole (23; 63, 64) is a long hole extending in the catheter long-axis direction which is formed by cutting away a side wall of the catheter main body forming the second lumen (16a; 66b), wherein a length of a major axis of the side hole (23; 63, 64) is at least 1.5 times an outer diameter of the catheter main body at a point where the side hole (23; 63, 64) is formed, and a length of a minor axis of the side hole (23; 63, 64) is between 0.5 and 0.9 times the outer diameter of the catheter main body; at a point where the side hole (23; 63, 64) is formed, when viewed from a first direction which is orthogonal to an opening direction of the side hole (23; 63, 64) and orthogonal to the catheter long-axis direction, an angle (θ1) formed by a tip-end opening edge of the side hole (23; 63, 64) and a catheter long-axis direction, and an angle (θ2) formed by a base-end opening edge of the side hole (23; 63, 64) and the catheter long-axis direction are both 35° or less; and characterised in that the catheter main body also has a closure member (31; 31A) having a tip-end side 32 having a shape with an outermost tapered surface 32a which is inclined with a reduced diameter toward the tip end for insertion into a blood vessel, and an insertion part 34 for insertion into the tip-end opening of the second lumen (16a; 66b), the insertion part 34 having a base-end side that extends to a position to seal the tip-end region of the side hole (23; 63, 64), and when viewed from the first direction (Al), a base-end surface of the closure member (31; 31A) is shaped to follow the shape of the tip-end opening edge of the side hole (23; 63, 64). the side hole (23; 63, 64) is a long hole extending in the catheter long-axis direction which is formed by cutting away a side wall of the catheter main body forming the second lumen (16a; 66b), wherein a length of a major axis of the side hole (23; 63, 64) is at least 1.5 times an outer diameter of the catheter main body at a point where the side hole (23; 63, 64) is formed, and a length of a minor axis of the side hole (23; 63, 64) is between 0.5 and 0.9 times the outer diameter of the catheter main body; at a point where the side hole (23; 63, 64) is formed, when viewed from a first direction which is orthogonal to an opening direction of the side hole (23; 63, 64) and orthogonal to the catheter long-axis direction, an angle (θ1) formed by a tip-end opening edge of the side hole (23; 63, 64) and a catheter long-axis direction, and an angle (θ2) formed by a base-end opening edge of the side hole (23; 63, 64) and the catheter long-axis direction are both 35° or less; and characterised in that the catheter main body also has a closure member (31; 31A) having a tip-end side 32 having a shape with an outermost tapered surface 32a which is inclined with a reduced diameter toward the tip end for insertion into a blood vessel, and an insertion part 34 for insertion into the tip-end opening of the second lumen (16a; 66b), the insertion part 34 having a base-end side that extends to a position to seal the tip-end region of the side hole (23; 63, 64), and when viewed from the first direction (Al), a base-end surface of the closure member (31; 31A) is shaped to follow the shape of the tip-end opening edge of the side hole (23; 63, 64).