The invention relates to a composter (1), comprising on top of each other a feeding zone (2), a composting zone (3) and a discharge zone (4). The feeding zone (2) is provided with means for feeding compostable mass into the composter. The discharge zone (4) is provided with means (17) for removing compost mass with a desired degree of ripeness from the composter. The composter (1) includes first separation means between the composting zone (3) and the discharge zone (4). Said first separation means comprise a first disc-shaped member (6) which is provided with at least one through slot (16), said slot having a cutting edge (160. The composter is provided with elements by means of which the first disc-shaped member is drivable for rotating the same around its center axis, whereby said first cutting edge (160 enables some mass to be cut off of the presently treated mass present above the first disc-shaped member (6), said cut-off mass being conveyable through said slot (16) into the discharge zone (4) underlying the first disc-shaped member (6). The invention relates also to a method for composting in a composter.