A diagnostic device for a battery includes a measurement unit configured to acquire a current value and a voltage value of the battery, and a diagnostic unit configured to calculate an internal resistance value of the battery based on the current value and the voltage value acquired by the measurement unit, and diagnose the battery based on the internal resistance value. The diagnostic unit is configured to cause the battery to perform discharge at a first current value and a second current value smaller than a predetermined target current value during a first period and a second period shorter than a predetermined target period, respectively, estimate the internal resistance value when the battery is caused to perform the discharge at the target current value during the target period, and diagnose the battery.