1. A target-image detecting device configured to detect a target image as a part of photographed images and as an image of a target object from the photographed images, the target-image detecting device comprising: a memory unit that stores information; an obtaining unit that sequentially obtains a plurality of the photographed images that form a moving image; a detecting unit that detects a target image included in the photographed image obtained by the obtaining unit, generates a detection result, and accumulates the generated detection result in the memory unit as detection hysteresis; and a deciding unit that: refers to the detection hysteresis of the memory unit, decides whether a detection result of a target image of the same target object is included in a latest predetermined number of detection results, outputs the detection result when the detection result is included, and does not output the detection result when the detection result is not included.