An oil type damper for braking and absorbing the opening-closing motion of a lid in a cassette tape recorder, video recorder or the like is improved by incorporating therein a flexible membrane formed in a structure symmetrical with respect to the obverse and reverse sides thereof, interposed between a housing and a braking disk, and adapted to absorb thermal expansion of oil and produce a sealing effect and, at the same time, causing the shaft projected from the braking disk for supporting the flexible membrane in place to contain a flared base radially diverged downwardly. Since the flexible membrane has a structure symmetrical with respect to the obverse and reverse sides thereof, the possibility of incorrect assembly of the oil type damper is minimized and the operational efficiency of damper production is increased without sacrificing sealing effect.