The electrically heated catalyst to which the present disclosure is applied is provided in a hybrid vehicle capable of switching its running mode between EV mode and HC mode and supplied with electrical energy before the internal combustion engine is started. An abnormality detection apparatus calculates an electrical energy parameter relating to the integrated value of electrical power actually supplied to the electrically heated catalyst (actually supplied electrical power) over a specific period of time from the time when supply of electrical power to the electrically heated catalyst is started and detects an abnormality of the electrically heated catalyst by comparing the electrical energy parameter with a threshold. The threshold is set according to the rate of decrease of the charge level of a battery (charge level decrease rate) during a period in which electrical power is supplied to the electrically heated catalyst.