1. An imaging device, comprising: image input means for inputting an image of a subject; image signal processing means for processing the input image; display means for displaying on a display screen an input image processed by the image signal processing means; face detection means for detecting a face of the subject from the input image processed by the image signal processing means; target face setting means for setting one of the faces, detected by the face detection means, as a smiling face detection target; display control means for displaying, on the input image displayed by the display means, the detected face set by the target face setting means in a manner such that the set detected face is distinctly different from the other detected faces; detection face changing means for changing the smiling face detection target to another detected face in response to a user operation performed on the display screen that displays the detected face set as the smiling face detection target and the other detected faces in the distinctly different manner; smiling degree detection means for detecting a degree of smile of the detected face as the smiling face detection target; and photographing control means for controlling a photographing operation in response to a smiling detection threshold level reached by the degree of smile detected by the smiling degree detection means, wherein when the face detection means detects a plurality of faces from the input image, the target face setting means determines an importance of each detected face based on a position and other information relating to the detected face in the input image, prioritizes the detected faces according to the importance of the respective detected faces, and sets as initial face detection targets each of the detected faces of a predetermined number starting with the one having the highest priority, when the detection face changing means identifies a detected face specified by the user operation, the target face setting means sets the user-specified detected face as one of the face detection targets having the highest priority, and when the number of the plurality of faces from the input image is greater than the predetermined number, and the user-specified detected face is not one of the initial face detection targets, the target face setting means removes a detected face in the initial face detection targets having the lowest importance as one of the face detection targets.