An object of the present invention is to provide a device used in an operation using a catheter assembly and the catheter assembly having the device, which allows a hole formed in a body surface to have a minimum size and a catheter to be smoothly inserted into the body. An auxiliary dilator includes a body part having a tapered portion tapering toward a front end of the auxiliary dilator. The body part surrounds an extension part of a main dilator with a front end entry portion of the extension part protruding from the tapered portion, and a separation part attached to the body part enabling the body part to be removed from the main dilator.
