A sealing structure is configured in the following manner. When a packing having a pair of sealing portions formed at opposite edge portions of the outer peripheral surface thereof is installed in a land portion of a spool, the outside diameter of the packing is made smaller than the diameter of a slide surface of a spool hole. In a state in which the outer periphery of the land portion opposes the slide surface and a pressurized fluid is supplied from a gas supply channel to the first space of first and second spaces that are partitioned by the land portion, the edge portion of the packing positioned near the first space is elastically elongated in a radial direction due to pressure of the pressurized fluid and thereby narrows a gap formed between the first sealing portion and the slide surface of the spool hole or abuts the slide surface.