An image diagnostic apparatus of an embodiment includes a density change acquisition unit, a trigger generating unit and a control unit. The density change acquisition unit is configured to acquire data corresponding to a temporal change in density of a contrast agent injected into an object. The trigger generating unit is configured to generate a trigger when abnormal data has been detected by a first threshold processing of the data. The trigger is generated by a second threshold processing of data other than the abnormal data. The control unit is configured to perform a control of contrast imaging for the object based on the trigger.