A filtration device (10) has a primary conveyer tank (11) which is equipped with a first conveyer (20), a primary drum filter (30) which rotates inside the primary conveyer tank (11), a secondary conveyer tank (40) which is equipped with a second conveyer (50) and a secondary drum filter (60) which rotates inside the secondary conveyer tank (40). Liquid (Q1) in the primary conveyer tank (11) passes through a first opening (41) to flow into the secondary conveyer tank (40). Clean liquid (Q2) flowing into a first clean tank (80) is supplied to a low-pressure-side supply system (2) by a first pump (85). Super-clean liquid (Q3) having been filtered by the secondary drum filter (60) passes through a second opening (91) to flow into a second clean tank (90), and then is supplied to a high-pressure-side supply system (3) by a second pump (95).