A magnetic separator for removing magnetic sludge from a liquid to be treated, comprising: a flow path for the liquid to be treated, the flow path extending from an upstream input part for inputting the liquid to be treated containing magnetic sludge to a treatment liquid discharge part for discharging a treatment liquid with the magnetic sludge removed therefrom; a main drum for discharging the magnetic sludge to the outside, the main drum being installed such that an approximately half the circumference on the lower side is immersed under the liquid surface of the liquid to be treated and an approximately half the circumference on the upper side comes out from the liquid surface; a first bottom wall installed below the main drum to be separated from the outer periphery of the main drum and with a shape along the outer periphery of the main drum; a sub-drum for magnetizing the magnetic sludge in the liquid to be treated to form magnetized aggregates, the sub-drum being disposed on the upstream side of the main drum and in a state of being immersed in the liquid to be treated; a second bottom wall installed below the sub-drum to be separated from the outer periphery of the sub-drum and with a shape along the outer periphery of the sub-drum, and a scraper for scraping off the magnetic sludge magnetically attracted to the sub-drum, the scraper being disposed to extend from the sub-drum to the main drum side, whereby one end of the scraper is in contact with the sub-drum, the other end is fixed to the first bottom wall, and an opening portion is formed on the other end of the scraper; wherein the flow path comprises an upper flow path through which the liquid to be treated flows above the sub-drum and the scraper, and a lower flow path through which the liquid flows below the sub-drum and the scraper, wherein, the opening portion is configured for sending the liquid in the lower flow path in the direction of rotation of the main drum, and the upper flow path and the lower flow path join to form a further flow path through which the liquid to be treated flows below the main drum so that the liquid to be treated, which is input from the upstream input part, is discharged to the treatment liquid discharge part through the sub-drum and the main drum. a flow path for the liquid to be treated, the flow path extending from an upstream input part for inputting the liquid to be treated containing magnetic sludge to a treatment liquid discharge part for discharging a treatment liquid with the magnetic sludge removed therefrom; a main drum for discharging the magnetic sludge to the outside, the main drum being installed such that an approximately half the circumference on the lower side is immersed under the liquid surface of the liquid to be treated and an approximately half the circumference on the upper side comes out from the liquid surface; a first bottom wall installed below the main drum to be separated from the outer periphery of the main drum and with a shape along the outer periphery of the main drum; a sub-drum for magnetizing the magnetic sludge in the liquid to be treated to form magnetized aggregates, the sub-drum being disposed on the upstream side of the main drum and in a state of being immersed in the liquid to be treated; a second bottom wall installed below the sub-drum to be separated from the outer periphery of the sub-drum and with a shape along the outer periphery of the sub-drum, and a scraper for scraping off the magnetic sludge magnetically attracted to the sub-drum, the scraper being disposed to extend from the sub-drum to the main drum side, whereby one end of the scraper is in contact with the sub-drum, the other end is fixed to the first bottom wall, and an opening portion is formed on the other end of the scraper; wherein the flow path comprises an upper flow path through which the liquid to be treated flows above the sub-drum and the scraper, and a lower flow path through which the liquid flows below the sub-drum and the scraper, wherein, the opening portion is configured for sending the liquid in the lower flow path in the direction of rotation of the main drum, and the upper flow path and the lower flow path join to form a further flow path through which the liquid to be treated flows below the main drum so that the liquid to be treated, which is input from the upstream input part, is discharged to the treatment liquid discharge part through the sub-drum and the main drum. the flow path comprises an upper flow path through which the liquid to be treated flows above the sub-drum and the scraper, and a lower flow path through which the liquid flows below the sub-drum and the scraper, wherein, the opening portion is configured for sending the liquid in the lower flow path in the direction of rotation of the main drum, and the upper flow path and the lower flow path join to form a further flow path through which the liquid to be treated flows below the main drum so that the liquid to be treated, which is input from the upstream input part, is discharged to the treatment liquid discharge part through the sub-drum and the main drum.