Abstract: A highly safe catheter assembly exhibiting excellent torque transmission, pushability and kink resistance and operation of a guidewire is provided. The catheter assembly comprises a catheter having a lumen extending between an opening on its distal end and an opening in its periphery in distal end portion of the catheter for insertion of a guidewire therethrough, and a covering sheath for covering said catheter together with the guidewire, said covering sheath having a guidewire passage means for enabling passage of said guidewire from its interior to its exterior. The catheter of this assembly may be a PTCA catheter.