A diagnostic device for a secondary battery having a structure in which positive electrodes and negative electrodes are alternately arranged includes an electronic control unit. The electronic control unit is configured to acquire a first resistance value indicating a magnitude of electrical resistance of the secondary battery, compress at least a part of the secondary battery, acquire a second resistance value indicating a magnitude of electrical resistance of the secondary battery after the compression, determine, using the first and second resistance values, whether an amount of decrease in electrical resistance of the secondary battery by the compression is greater than a predetermined value, and determine whether an increase in resistance due to distortion of at least one of the positive electrodes and the negative electrodes occurs in the secondary battery using a result of the determination.