To provide a fuel oil cleaning system of a diesel engine which can reduce a concentration of a residue in a fuel circulation route and suppress waste fuel.

A filter size of elements 21, 33 of a two-step type filter 16 is 10 μm or less so that a residue having a size of 10 to 30 μm is not shut in a circulation route consisting of the two-step type filter 16 and a fuel oil supply line Lf. That is, a burden of a first step filter 17 is reduced because a concentration of a residue in the two-step type filter 16 is reduced. The element 33 of a second step filter 23 is backwashed using fuel oil filtered by the first step filter 17 to prevent clogging of the element 33. The fuel oil obtained by separating residual oil on inlet side of the element 33 of the second step filter 23 by a centrifugal separator 40 is returned to the fuel oil supply line Lf to prevent waste fuel oil.