[Object] In view of a perfect seal being less necessary for a contaminant seal than for a fluid seal, it is an object of the present invention to use a seal ring having a seal part configured so that a contact area of the seal part is reduced and the seal part will not be compromised under pressure of a sealed fluid, whereby a contaminant-sealing device that takes up less space, and that operates with less friction, is provided. [Means for solving the problems] A contaminant-sealing device according to the present invention is characterized in that , in a sealing device comprising a rotating or reciprocatingly moving seal ring attached to one of two mutually opposed members and slidably placed in intimate contact with the other member, the seal ring has a cross-sectional shape that is quadrilateral and has a taper so that a surface of a sliding part is lower towards a high-pressure side and higher towards a low-pressure side, [the seal ring] being disposed so that a fluid body on the high-pressure side acts on the tapered part.