Diagnostic accuracy is improved when abnormality diagnosis of a particulate filter is carried out by using a PM sensor arranged in an exhaust passage downstream of the particulate filter. An abnormality diagnostic device for a particulate filter includes a PM sensor arranged in an exhaust passage at the downstream side of the particulate filter so as to output a signal corresponding to an amount of PM deposited in between electrodes which constitute a sensor element, and a filter diagnostic unit to carry out filter diagnostic processing in which abnormality of the particulate filter is diagnosed based on an output value of the PM sensor, wherein it is decided whether to carry out the filter diagnostic processing based on a degree of variation in the output value of the PM sensor before the filter diagnostic processing is carried out.