Failure determination device for an emission control device of an internal combustion engine

When the individual control of a plurality of intake valves is not applicable, it is an object of the present invention with high accuracy to determine which is abnormal from the plurality of supply valves, while an increase in cost is suppressed. A first supply valve, a first selective NOx reduction catalyst, a second supply valve, a second selective NOx reduction catalyst and an NOx sensor are sequentially provided in an exhaust pipe. Regarding the identification of which first from the supply valve and the second intake valve is abnormal, an instruction to the first supply valve and the second intake valve is output to increase a supply amount of a reducing agent. This identification is based on a detected by the NOx sensor according to the lapse of a first predetermined time period since an instruction time point, which is a time when the instruction is output, NOx concentration.