Biological treatment device and biological treatment method

Provided is a biological treatment device capable of suppressing adhesion of a carrier to a screen. This biological treatment device is provided with: a first biological treatment tank in which a carrier is flowed, and microorganisms for biologically treating water to be treated are immobilized on the carrier; a partition wall provided in the first biological treatment tank and separating the first biological treatment tank into a first region including a supply unit for water to be treated, and a second region communicating with the first region at the lower part of the first biological treatment tank and having an outflow unit for treated water treated in the first biological treatment tank formed at the upper part; and a carrier capturing device provided with a screen provided in the outflow part, the screen preventing the carrier from flowing out of the first biological treatment tank, the carrier being filled in the first region, the first biological treatment tank having a first side wall facing the par