A diagnosis apparatus for reducing agent supply apparatus that diagnoses a reducing agent supply apparatus having a reducing agent addition valve that is provided in an exhaust system of an internal combustion engine and injects liquid reducing agent into exhaust gas, comprising: temperature sensing means for sensing the temperature of the exhaust gas, provided at a position in the exhaust system of the internal combustion engine at which spray of reducing agent is formed as the reducing agent is injected through said reducing agent addition valve; temperature decrease calculation means for calculating the decrease in the temperature of the exhaust gas sensed by the temperature sensing means as the reducing agent is injected through the reducing agent addition valve; and diagnosis means for diagnosing whether or not there is a malfunction in said reducing agent supply apparatus based on the decrease in the temperature of the exhaust gas calculated by the temperature decrease calculation means, the diagnosis apparatus being characterized by further comprising: reducing agent injection quantity estimation means for estimating the quantity of reducing agent actually injected through said reducing agent addition valve based on the decrease in the temperature of the exhaust gas calculated by said temperature decrease calculation means; wherein said diagnosis means determines that there is a malfunction in said reducing agent supply apparatus if the difference between a quantity of reducing agent that should be injected through said reducing agent addition valve and the quantity of reducing agent estimated by the reducing agent injection quantity estimation means is larger than a threshold value at which it can be determined that said reducing agent supply apparatus is normal. temperature sensing means for sensing the temperature of the exhaust gas, provided at a position in the exhaust system of the internal combustion engine at which spray of reducing agent is formed as the reducing agent is injected through said reducing agent addition valve; temperature decrease calculation means for calculating the decrease in the temperature of the exhaust gas sensed by the temperature sensing means as the reducing agent is injected through the reducing agent addition valve; and diagnosis means for diagnosing whether or not there is a malfunction in said reducing agent supply apparatus based on the decrease in the temperature of the exhaust gas calculated by the temperature decrease calculation means, the diagnosis apparatus being characterized by further comprising: reducing agent injection quantity estimation means for estimating the quantity of reducing agent actually injected through said reducing agent addition valve based on the decrease in the temperature of the exhaust gas calculated by said temperature decrease calculation means; wherein said diagnosis means determines that there is a malfunction in said reducing agent supply apparatus if the difference between a quantity of reducing agent that should be injected through said reducing agent addition valve and the quantity of reducing agent estimated by the reducing agent injection quantity estimation means is larger than a threshold value at which it can be determined that said reducing agent supply apparatus is normal. reducing agent injection quantity estimation means for estimating the quantity of reducing agent actually injected through said reducing agent addition valve based on the decrease in the temperature of the exhaust gas calculated by said temperature decrease calculation means; wherein said diagnosis means determines that there is a malfunction in said reducing agent supply apparatus if the difference between a quantity of reducing agent that should be injected through said reducing agent addition valve and the quantity of reducing agent estimated by the reducing agent injection quantity estimation means is larger than a threshold value at which it can be determined that said reducing agent supply apparatus is normal.