A blood vessel dilator so thin in outside dameter as the "on the wire" type catheter retaining a sufficiently wide liquid lumen and highly manipulatable at branched parts of blood vessels by a separately turnable guide wire and hence readily inserted even into narrow perpheral blood vessels.This blood vessel dilator comprises a main tube; a distal-end tube put in the distal end portion of the main tube with its front end portion protruding out of the distal end of the main tube; a dilatation element with its front end portion connected to the front end portion of the distal-end tube and its rear end portion to the distal end portion of the main tube; a resilient core member put through the inside of the main tube and distal-end tube; a leading guide provided at the distal end portion of the resilient core member; a main-tube hub connected to the proximal end of the main tube; and a core-member hub connected to the proximal end of the resilient core member and secured to the main-tube hub turnably with respect to the main tube.