The present invention improves the detection accuracy of the state of charge of a battery. A coulomb count value CC is generated by integrating a charge/discharge current IBAT of a battery (S100). An SOC value SOC1 is calculated (S102). Based on an SOC-OCV characteristic predetermined for the battery, an OCV value OCV1 corresponding to the value SOC1 is generated (S112). A voltage VBAT of the battery is detected (S104). A voltage drop VDROP1 between OCV1 and a detected value VBAT1 of the voltage VBAT is generated (S114). A value OCV2 greater than the minimum operating voltage of the system by ΔV, which corresponds to the voltage drop VDROP1, is generated (S116). Based on the SOC-OCV characteristic, an SOC value SOC2 corresponding to OCV2 is generated (S118). Based on the value SOC2, at least one of the values SOC1, CC, CCFULL, and SOC-OCV characteristics is corrected.