A rotary drum type device for separating solid particles from a liquid includes a rotary screen drum having support rods extending in the axial direction and arranged cylindrically and having a projecting portion in a radially outward end portion, and a wedge wire wound spirally on the outer periphery of the support rods in substantially crossing direction to the support rods. The wedge wire is arranged with its one side facing outward and two other sides forming a slit which widens radially inwardly between adjacent wedge wire portions and with an inward apex of said wedge wire being welded to the projecting portion of the support rods at crossing points. The rotary drum type device further includes a container containing a liquid to be treated disposed outside of the rotary screen drum, a suction device connected to the rotary screen drum for reducing pressure in the rotary screen drum, and a scraper for stripping off cake produced on the outer periphery of the rotary screen drum. The width of the slit of the wedge wire is within a range between 1 micron and 150 microns.