A diesel particulate filter (14) which traps particulate matter and a NOx
trap catalyst (13) which traps nitrogen oxides are installed in series in
an exhaust passage (10) of a diesel engine (1). A controller (21)
calculates a particulate matter deposition amount in the diesel
particulate filter (14) (S4). The controller (21), when the particulate
matter deposition amount exceeds a first predetermined amount (PM2),
prevents sharp increase of the particulate matter deposition amount by
prohibiting rich spike where the oxygen concentration of the exhaust gas
is controlled to a value corresponding to a rich air-fuel ratio in order
to regenerate the NOx trap catalyst (13) (S301, S302, S306), and prevents
the particulate matter deposition amount of the diesel particulate filter
(14) from reaching a limit.