Provided is a clogging determining device and a clogging determining method for a reducing agent injection valve, which enable clogging of the reducing agent injection valve to be determined with high precision in further consideration of the individual difference of the system. There are provided an opened-valve pressure reduction amount calculating element for calculating an opened-valve pressure reduction amount representing a pressure reduction amount in the supply passage which is reduced in a predetermined time when the pressure-feeding operation of the pump is stopped under the state that the reducing agent injection valve is set to an injection mode, a closed-valve pressure reduction amount calculating element for calculating a closed-valve pressure reduction amount representing the pressure reduction amount in the supply passage which is reduced in a predetermined time when the pressure-feeding operation of the pump is stopped under the state that the reducing agent injection valve is set to a full-close mode; and a clogging determining element for comparing the opened-valve pressure reduction amount with the closed-valve pressure reduction amount to determine clogging of the reducing agent injection valve.