A power supply system comprising a power supply state monitoring device, an apparatus, a first power supply apparatus, and a second power supply apparatus; the power supply state monitoring device comprising: a determination unit configured to, when power is supplied from the first power supply apparatus including an AC cord to a main body of the apparatus in which the first power supply apparatus and the second power supply apparatus including an AC cord can be mounted, power being supplied from the second power supply apparatus to the main body of the apparatus, the state of the power supplied from the second power supply being monitored, make a first determination based on a power-supply mount-state notification signal (INST-1) which is output from the second power supply apparatus and indicates whether or not the second power supply apparatus is mounted in the main body of the apparatus, and if the result of the first determination indicates that the second power supply apparatus is mounted in the main body of the apparatus, the determination unit is configured to make a second determination based on an AC-supply state notification signal (INST) which is output from the second power supply apparatus and indicates whether or not the power is supplied from the AC cord of the second power supply apparatus; and a notification unit configured to, if the result of the first determination indicates that the second power supply apparatus is not mounted in the main body of the apparatus, notify a user of the fact that the second power supply apparatus is not mounted in the main body of the apparatus, and if the result of the second determination indicates that the power is not supplied from the AC cord of the second power supply apparatus, the notification unit is configured to notify the user of the fact that the second power supply apparatus is mounted in the main body of the apparatus and the power is not supplied from the AC cord of the second power supply apparatus, wherein the power supply state monitoring device is mounted in the main body of the apparatus. a determination unit configured to, when power is supplied from the first power supply apparatus including an AC cord to a main body of the apparatus in which the first power supply apparatus and the second power supply apparatus including an AC cord can be mounted, power being supplied from the second power supply apparatus to the main body of the apparatus, the state of the power supplied from the second power supply being monitored, make a first determination based on a power-supply mount-state notification signal (INST-1) which is output from the second power supply apparatus and indicates whether or not the second power supply apparatus is mounted in the main body of the apparatus, and if the result of the first determination indicates that the second power supply apparatus is mounted in the main body of the apparatus, the determination unit is configured to make a second determination based on an AC-supply state notification signal (INST) which is output from the second power supply apparatus and indicates whether or not the power is supplied from the AC cord of the second power supply apparatus; and a notification unit configured to, if the result of the first determination indicates that the second power supply apparatus is not mounted in the main body of the apparatus, notify a user of the fact that the second power supply apparatus is not mounted in the main body of the apparatus, and if the result of the second determination indicates that the power is not supplied from the AC cord of the second power supply apparatus, the notification unit is configured to notify the user of the fact that the second power supply apparatus is mounted in the main body of the apparatus and the power is not supplied from the AC cord of the second power supply apparatus, wherein the power supply state monitoring device is mounted in the main body of the apparatus.