the sensor value of the exhaust gas temperature sensor the in pipe injection amount in the forced regeneration can be optimized by this embodiment because the doc internal temperature t doc is accurately detected accordingly the exhaust purification system of this embodiment enables the fuel injection amount in the forced regeneration to be controlled with accuracy and can thus effectively improve the regeneration efficiency of the dpf 22 in addition an exhaust gas temperature sensor s do es not need to be provided across the doc 21 therefore the cost and the size of the overall apparatus can be effectively reduced it should be noted that the present invention is not limited to the above described embodiment and can be implemented with modifications as appropriate within the scope that does not depart from the spirit of the present invention for example it suffices that the number of pairs of the electrodes 27 is at least one and the illustrated embodiment is not limiting in this regard the engine 10 is not limited to a diesel engine and an embodiment can be applied widely to other internal combustion engines including a gasoline engine as illustrated in fig8 a bypass passage 25 may be connected to the exhaust passage 12 so as to bypass the doc 21 and the doc 21 a for measurement may be disposed in the bypass passage 25 the doc 21 a has a small capacity in this configuration electrodes 27 are preferably provided in the doc 21 a and an orifice 25 a restriction for regulating the flow rate of the exhaust gas is preferably provided in the bypass passage 25 when a forced regeneration of the doc 21 a is.