A seal structure in which a sealing member 510 fitted in the seal holding part 120 is elastically deformed to hermetically seal a flow passage joining portion. The seal holding part 120 opening on a flow passage side and including a first retaining surface 121, a second retaining surface 122, and a circumferential surface 123. The sealing member 510 includes a first surface 511 in contact with the first retaining surface 121, a second surface 512 in contact with the retaining surface 122, and an inner surface 513 located inside the seal holding part 510 and tapered to have a diameter becoming smaller from the first surface 511 side to the second surface 512 side. An engagement portion 515 engaged in the seal holding part 120 is formed protruding from an outer surface 514 located on the circumferential surface 123 side and on the first surface 511 side. When the sealing member 510 is fitted in the seal holding part 120 while the engagement portion 515 is pressed, the sealing member 510 is placed such that the inner surface 513 on the first surface 511 side protrudes inward and the outer surface 514 is in contact with the circumferential surface 123.