According to one embodiment, a device state estimation apparatus includes a detector and an operating state estimator. The detector detects devices in the ON operating state, based on a comparison between a calculated one of a first characteristic of a current or power harmonic in a power feed line and a reference one of the first characteristic which has been predetermined for at least one device. The operating state estimator extracts at least one set of the second characteristics from plural sets of second characteristics associated with harmonic, which have been predetermined. Each set of the second characteristics is for operating states of each of the devices. The one set of the second characteristics is for operating states of the device in the ON operating state which is detected by the detector. The operating state estimator estimates the operating state of the device based on the similarity between the extracted one of the second characteristics and a calculated one of the second characteristics obtained from the current or power harmonic.