Provided is a structure for an external cover of a replaceable cartridge used in an air-drying device in a vehicle compressed-air brake device or the like. Said structure improves the seal between the replaceable cartridge and a base, improving the quality and reliability of the air-drying device. Specifically, a first inclined section (16g) is formed at a position midway between the end (16e) of a raised piece (16a) and the base (16f) of an external cover (16), and a left-side vertical wall section (16h) is raised from the base (16f). A right-side vertical wall section (16i) is formed opposite the left-side vertical wall section (16h). A top surface (16j) is formed between the left-side and right-side vertical wall sections, a recess (16b) is formed from the left-side vertical wall section (16h) and the right-side vertical wall section (16i) and top surface (16j), and a sealing member (21) is fitted inside. The bottom edge of the right-side vertical wall section (16i) forms an arc-shaped section (16k). A second inclined section (16n) is extended at a prescribed height (H2) from a bottom edge surface (16m), at a position midway along the height (H1) of the external cover (16). An outermost vertical wall section (16q) is lowered from the second inclined section (16n), with an arc-shaped section (16p) in between.