A method for manufacturing a flow path device internally provided with a flow path for allowing a liquid to flow, the method comprising: a coating film formation step of preparing two or more members (10,11) as members constituting the flow path device, with at least one of the members being provided with a groove corresponding to the flow path, and forming a hydrophilic coating film in the at least one member of the members, using a treatment liquid including a hydrophilizing agent, the hydrophilic coating film covering a surface of the at least one member at a side to be joined to another member; an irradiation step of selectively irradiating only a joining surface of the hydrophilic coating film with ultraviolet rays or plasma derived from an oxygen-containing gas in the member having the hydrophilic coating film, thereby removing the hydrophilic coating film in ' the joining surfaces and activating a surface of the joining surface, and wherein in the irradiation step, the ultraviolet rays have a wavelength of 254 nm or less, and an exposure amount of the irradiation is 400 mJ/cm 2 or more and 5000 mJ/cm 2 or less; a compression bonding step of applying an external force to the two or more members, (10,11) disposed in predetermined positions such that the joining surfaces of the two or more members face each other, so as to compression bond the joining surfaces, with the joining surfaces having been irradiated with the ultraviolet rays or the plasma and activated, wherein a material of the surfaces of the joining surfaces of the two or more members (10,11) is a material that can be activated by the ultraviolet rays or the plasma, and wherein the another member seals an opening of the groove of the at least one member so as to form the flow path. a coating film formation step of preparing two or more members (10,11) as members constituting the flow path device, with at least one of the members being provided with a groove corresponding to the flow path, and forming a hydrophilic coating film in the at least one member of the members, using a treatment liquid including a hydrophilizing agent, the hydrophilic coating film covering a surface of the at least one member at a side to be joined to another member; an irradiation step of selectively irradiating only a joining surface of the hydrophilic coating film with ultraviolet rays or plasma derived from an oxygen-containing gas in the member having the hydrophilic coating film, thereby removing the hydrophilic coating film in ' the joining surfaces and activating a surface of the joining surface, and wherein in the irradiation step, the ultraviolet rays have a wavelength of 254 nm or less, and an exposure amount of the irradiation is 400 mJ/cm 2 or more and 5000 mJ/cm 2 or less; a compression bonding step of applying an external force to the two or more members, (10,11) disposed in predetermined positions such that the joining surfaces of the two or more members face each other, so as to compression bond the joining surfaces, with the joining surfaces having been irradiated with the ultraviolet rays or the plasma and activated, wherein a material of the surfaces of the joining surfaces of the two or more members (10,11) is a material that can be activated by the ultraviolet rays or the plasma, and wherein the another member seals an opening of the groove of the at least one member so as to form the flow path.