To facilitate current measurement in sleep sections without requiring a measuring instrument with high resolution accuracy, in a portable electronic apparatus with a sleep mode for power saving.

A control unit 17 adds a current value actually measured during non-sleep to an accumulated value of the current value during the sleep and for the sleep sections based on a reference capacity table stored in a storage section 16 in advance and on the current value in the sleep sections measured by a measurement instrument connected to outside, and subtracts the added value from the reference capacity value stored in a reference capacity table 160 to calculate a battery residual amount.