A flow passage sealing structure for omitting a process of welding or caulking an orifice plate and a filter plate to an orifice base and a filter base as base materials and allowing further miniaturization, includes a main block (1) including main flow passages (1a, 1b), recessed portions (12, 13) provided in side surfaces of the main block and having female screws in inner peripheral surfaces, thin plates (6, 8) abutting against the bottom surfaces of the recessed portions and having through holes, gasket rings (16, 17) abutting against the thin plates (6, 8), pressing pipelines (20, 21) having large-diameter portions and internal flow passages communicable with the main flow passages (1a, 1b) and abutting against the gasket rings, and fastening screws (22) abutting against the large-diameter portions and pressing the pressing pipelines by being inserted around the outside of the pressing pipelines and screwed into the female screws.