A flow path groove is formed in the surface of at least one of two resin substrates; the two resin substrates are joined with the surface in which the flow path groove is formed facing inward; a through-hole having a substantially round cross section is formed in either one of the two resin substrates such that the through-hole connects with the flow path groove from the surface opposite the surface where the two resin substrates join; protruding parts, which protrude in the direction of thickness of the resin substrates and are disposed enclosing the through-hole, are formed in the surface on the opposite side; a space, which has a substantially round cross-sectional shape concentric with the through-hole and has depth in the same direction as the direction of thickness of the resin substrates, is formed in the joined surfaces when the protruding parts are projected from the direction substantially perpendicular to the joined surfaces; and the correlation of fc>fa is satisfied when the edges on the base end side of the through-hole are projected onto the joined surfaces from a substantially perpendicular direction, where the diameter on the base end side of the through-hole is fa and the diameter on the end edge side of the space touching the joined surfaces is fc