A gasket sandwiched between a flow cell and a joint member has, in a main flat surface, an inlet communication hole for communicating between an inlet hole of the flow cell and an inlet flow path of the joint member and an outlet communication hole for communicating between an outlet hole of the flow cell and an outlet flow path of the joint member. Furthermore, a deformation absorbing structure for absorbing deformation of the gasket is provided between the inlet communication hole and the outlet communication hole in the main flat surface of the gasket.