There are provided a clogging determining device for a reducing agent passage and a clogging determining method for a reducing agent passage, which can accurately detect clogging and estimate a clogged place when clogging occurs in any place of a supply line or a return line.The device includes pump driving control element for subjecting the pump to DUTY control so that a pressure value is kept to a predetermined value, driving DUTY determining element for determining whether the driving DUTY of the pump is less than a predetermined threshold value corresponding to the opening degree of the reducing agent injection valve, or not; pressure reduction amount calculating element for calculating a pressure reduction amount by which the pressure value detected by the pressure sensor is reduced within a predetermined time after the pump is stopped and the reducing agent injection valve is fully opened when it is determined that the driving DUTY of the pump is less than the threshold value, and clogging determining element for determining on the basis of the pressure reduction amount calculated by the pressure reduction amount: calculating element whether clogging occurs in the first reducing agent passage or the second reducing agent passage.