A malfunction diagnosis apparatus for use with an exhaust emission control system of an internal combustion engine, the exhaust emission control system including a reducing agent supply device, a NOx selective catalytic reduction catalyst and a NOx sensor, the reducing agent supply device being provided in an exhaust passage of the internal combustion engine, the reducing agent supply device being configured to supply ammonia precursor or ammonia into the exhaust passage as reducing agent, the NOx selective catalytic reduction catalyst being provided in the exhaust passage downstream of the reducing agent supply device in a flow direction of exhaust gas, the NOx selective catalytic reduction catalyst being configured to selectively reduce NOx by using reducing agent that is adsorbed on the NOx selective catalytic reduction catalyst, the NOx sensor being provided in the exhaust passage downstream of the NOx selective catalytic reduction catalyst in the flow direction of exhaust gas, the NOx sensor being configured to detect NOx and ammonia, the malfunction diagnosis apparatus characterized by comprising an electronic control unit configured to: carry out a diagnosis of whether there is a malfunction in the reducing agent supply device based on a detected value of the NOx sensor; estimate an estimated amount of adsorption when the reducing agent supply device is normal, the estimated amount of adsorption being an estimated amount of reducing agent adsorbed on the NOx selective catalytic reduction catalyst; estimate a corresponding detected value, the corresponding detected value being a detected value of the NOx sensor, the detected value of the NOx sensor corresponding to the amount of reducing agent adsorbed on the NOx selective catalytic reduction catalyst; and wherein the electronic control unit is configured to prohibit the diagnosis based on the detected value of the NOx sensor when the estimated amount of adsorption is larger than or equal to a first amount of adsorption and smaller than or equal to a second amount of adsorption, the first amount of adsorption being an amount of adsorbed reducing agent corresponding to a minimum value of the corresponding detected value, the second amount of adsorption being an amount of adsorbed reducing agent corresponding to the corresponding detected value same as a first detected value, the first detected value being the corresponding detected value when the amount of adsorbed reducing agent is zero, wherein the second amount of adsorption is larger than the first amount of adsorption. carry out a diagnosis of whether there is a malfunction in the reducing agent supply device based on a detected value of the NOx sensor; estimate an estimated amount of adsorption when the reducing agent supply device is normal, the estimated amount of adsorption being an estimated amount of reducing agent adsorbed on the NOx selective catalytic reduction catalyst; estimate a corresponding detected value, the corresponding detected value being a detected value of the NOx sensor, the detected value of the NOx sensor corresponding to the amount of reducing agent adsorbed on the NOx selective catalytic reduction catalyst; and wherein the electronic control unit is configured to prohibit the diagnosis based on the detected value of the NOx sensor when the estimated amount of adsorption is larger than or equal to a first amount of adsorption and smaller than or equal to a second amount of adsorption, the first amount of adsorption being an amount of adsorbed reducing agent corresponding to a minimum value of the corresponding detected value, the second amount of adsorption being an amount of adsorbed reducing agent corresponding to the corresponding detected value same as a first detected value, the first detected value being the corresponding detected value when the amount of adsorbed reducing agent is zero, wherein the second amount of adsorption is larger than the first amount of adsorption.