A battery-state monitoring apparatus includes: a secondary battery 200 supplying an electric power to portable equipment 300; and an operation processing part 50 that can detect a battery state of the secondary battery 200 in a detection mode to detect the battery state of the secondary battery 200, the operation processing part using the secondary battery 200 as a power source, wherein the battery-state monitoring apparatus includes a start-up current detecting part 31 detecting a start-up of the portable equipment 300, and wherein the operation processing part 50 waits for a detection of the battery state in a standby mode in which power consumption is smaller than the detection mode until the start-up is detected by the start-up current detecting part 31, and, on the other hand, the operation processing part 50 detects, by intermittently performing a temporary return from the standby mode to the detection mode, the battery state of the secondary battery in a return period during which the return is made.