An assembled battery monitoring apparatus includes: a first operation functioning device that carries out a monitoring process on an assembled battery configured by a plurality of battery cells connected in series and communicates with a host controller only in an ordinary operational timing in which the host controller controls an external power source to supply an operation power source; a second operation functioning device that carries out the monitoring process on the assembled battery in the ordinary operational timing and in a non-ordinary operational timing in which the external power source stops to supply the operation power source; and a power supply switching device that performs switching so as to enable the assembled battery to supply the operation power source in the non-ordinary operational timing.