1. An imaging device comprising: an image capturing part to capture frames of an image continuously; a first detection part to detect one or more subjects in the image captured by the image capturing part; a second detection part to follow and detect the one or more subjects detected by the first detection part; and a control part to cause, after the first detection part detects the one or more subjects in a first region of a first frame of the image, the second detection part to follow and detect the one or more subjects in a second and subsequent frames of the image, and including a setting part to set a part of the image as a limited region which is smaller than the first region of the image, and configured to cause the first detection part to perform subject detection in the limited region while shifting the limited region in position over the second and subsequent frames of the image, and wherein the control part of the imagining device is configured such that the subject detection that is performed by the first detection part in the limited region occurs subsequent to the detection of the one or more subjects in the first region; and wherein the setting part determines, according to a focal length, a frequency with which the region including the center of the image is set as the limited region.