To obtain a filtering device which enables long-term continuous operation, improves back wash effect of a filter, reduces the amount of discarded fluid in back washing, decreases the frequency of the back washing, and simplifies the structure thereof.

When inflow valves 9A, 9B are opened and discarding valves 17A, 17B are closed, heavy oil flows in the forward direction through two filters 21A, 21B provided in parallel in a main passage. As a result, normal filtration operation is performed. Occasionally, the back washing of the filters 21A, 21B is performed during the normal filtration operation. The time when the back washing is started is determined based on the pressure difference of the heavy oil between an inflow port 3 and an outflow port 29. When the inflow valve 9A is closed and the discarding valve 17A is opened, the heavy oil flows through the filter 21A in the opposite direction, whereby the back washing of the filter 21A is performed. The filter 21B continues the filtration operation during the back washing of the filter 21A. Subsequently the back washing of the filter 21B is performed.