A catheter and a manufacturing method thereof are disclosed, which can restrain a guide wire lumen and a dilation lumen from communicating with each other during thermal fusion-bonding while suppressing an increase in an outer diameter of a catheter shaft. A catheter has a catheter shaft in which a distal side shaft, an inner tube shaft, and a proximal side shaft are integrated with each other in a fusion region. The inner tube shaft has an opening portion which, opens outward and an arc-shaped portion which extends from the opening portion to a proximal side and which has only a portion of an entire circumference in a circumferential direction of the inner tube shaft. The fusion region includes the opening portion located on a distal side of a proximal end of the distal side shaft.