A catheter assembly includes a catheter having a main lumen and a sub lumen and an inner needle inserted through and disposed in the main lumen and allowing blood to flow in the proximal direction of the catheter from a needle tip exposed at the distal end of the catheter. The gap between an outer peripheral surface of the inner needle and a first inner peripheral surface of the main lumen continuous with the distal end of the catheter is narrower than the gap between the outer peripheral surface of the inner needle and a second inner peripheral surface closer to the proximal end side than the first inner peripheral surface.
