A catheter assembly (10) is inserted into a catheter (30) having a main lumen (34) and a sub lumen (41) and is inserted into a main lumen (34), and the blood can flow from the needle tip exposed at the tip of the catheter (30) to the proximal end of the catheter An inner needle (12) is provided.The distance between the inner peripheral surface (36) of the main lumen (34) leading to the distal end of the catheter (30) and the outer peripheral surface (16) of the inner needle (12) is narrower than the distance between the inner peripheral surface (38) of the proximal end side of the distal end (36) and the outer peripheral surface (16) of the inner needle (12).カテーテル組立体（１０）は、メインルーメン（３４）及びサブルーメン（４１）を有するカテーテル（３０）と、メインルーメン（３４）に挿通及び配置され、カテーテル（３０）の先端に露出される針先からカテーテル（３０）の基端方向に血液を流動可能な内針（１２）とを備える。カテーテル（３０）の先端に連なるメインルーメン（３４）の先端内周面（３６）と内針（１２）の外周面（１６）との間隔は、先端内周面（３６）よりも基端側の基本内周面（３８）と内針（１２）の外周面（１６）との間隔よりも狭い。
