According to one embodiment, a storage device includes, when power is supplied to a storage unit, counting of an elapsed time is started. If a command is input from a host device, and the elapsed time from input of a previous command to input of a current command is calculated based on time information clocked by the host device and on a counter value counted until the corresponding command is input. Matching of the time information is determined based on a temporal relation between the adding result of adding the calculated elapsed time to the time information included in the previous command and the time information included in the current command. When the mismatching is determined, data in the storage unit is invalidated.