A device for detecting deterioration of a catalyst in an internal combustion engine having a heater on the upstream side of a catalytic converter provided in an exhaust gas passage, the device being capable of expanding the deterioration determination conditions of the catalyst. It is determined whether the conditions, except the temperature condition, are all obtained among the conditions for detecting deterioration of the catalytic converter. When the conditions, except the temperature condition, are all obtained, an electric power is supplied from a power source to the heater to satisfy the temperature condition. Deterioration of the catalyst can be reliably detected if the electric power is supplied to the heater only when the engine is in the idling operation condition and if deterioration of the catalyst is detected when the time for supplying electric power to the heater has become larger than a predetermined period of time. The heater may be one contained in the electrically heated catalyst.