A battery diagnosis apparatus includes an SOC calculator, a ΔSOC obtaining unit, a largest block identification unit, and a diagnosis unit. The SOC calculator calculates an SOC for each parallel cell block included in a battery assembly based on an output from a battery sensor that detects a state of the battery assembly. When charging or discharging of the battery assembly is carried out, the ΔSOC obtaining unit obtains ΔSOC for each parallel cell block. The largest block identification unit identifies a largest block in a diagnosis target. When a degree of deviation between ΔSOC of the largest block and a ΔSOC reference value exceeds a prescribed level, the diagnosis unit determines that abnormality has occurred in the largest block in the diagnosis target.