To provide a novel baloon catheter, and a catheter auxiliary device to be used for that in which an outer diameter of an outer tube for composing a catheter tube can be shortened to reduce burden on a patient while improving baloon part dilation/contraction responsiveness.

This device comprises an outer tube 8 having a first lumen 12 inside, an inner tube 10 having a second lumen 14 inside axially extended inside the first lumen 12 in the outer tube 8 and protruded to the remote distal end side than the distal end side of the outer tube 8, a baloon part 4 of which proximal end part is jointed with a remote distal end of the outer tube 8 to form a baloon space communicated with the first lumen 8 inside, of which remote distal end is jointed with the remote distal end of the inner tube 10, and in which pressure fluid is introduced and discharged to/from the baloon space through the first lumen 12 to make an expanded condition and a contracted condition, and a valve element 60 installed in the second lumen 14 to be capable of sealing inner parts on the proximal end side of the second lumen 14 at least.