PROBLEM TO BE SOLVED: To provide a catheter and a catheter assembly, preventing a back shadow by a guide wire from occurring in an image when imaging the image inside a living body lumen.SOLUTION: This catheter 2 includes: a flexible elongated catheter body 21 and a connector part 22 fixed to a base end part of the catheter body 21. The catheter body 21 is formed with a sensor lumen 211 into which an imaging core 3 is inserted, and a guide wire lumen 212 into which the guide wire 200 is inserted, along the longitudinal direction of the catheter body 21. The guide wire lumen 212 is formed in only a tip part of the catheter body 21. The base end of the guide wire lumen 212 is extendedly provided on the base end side of the catheter body 21 rather than the base end of an imaging region 27.
