A horizontal centrifugal separator is configured such that a groove formed along a joint surface between an upper casing and a lower casing constituting the casing is outwardly bent near an opening of the casing, and that an elastic sealing member is disposed in the groove and an end portion thereof abuts against a sealing surface provided on a ring member which is provided so as to surround the outer circumference of the opening of the casing and is pressed against the casing. In this case, surface sealing is constituted by the end portion of the elastic sealing member and the sealing surface of the ring member. In this manner, the hermeticity can be improved without using a sealing member having a complicated structure.