There are provided a catheter and a manufacturing method thereof 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 10 has a catheter shaft 11 in which a distal side shaft 20, an inner tube shaft 22, and a proximal side shaft 21 are integrated with each other in a fusion region 25 by means of fusing. The inner tube shaft 22 has an opening portion 11a which opens outward and an arc-shaped portion 40 which extends from the opening portion 11a to a proximal side and which has only a portion of an entire circumference in a circumferential direction of the inner tube shaft 22. The fusion region 25 includes the opening portion 11a located on a distal side of a proximal end of the distal side shaft 20. A region in which the arc-shaped portion 40 and the proximal side shaft 21 are fused with each other is formed in the fusion region 25 on a proximal side of the opening portion 11a. The proximal side shaft 21 and the inner tube shaft 22 are fused with each other over an entire length from a proximal portion to a distal portion of the fusion region 25.