1. A target image detection device for detecting all target images from a plurality of sequential photographed images of which a motion picture is composed, the target images being regions of the sequential photographed images that each include all or a predetermined part of an unspecified and undetermined number of subjects, the target image detection device comprising: a processor; a memory coupled to the processor; and a condition storing unit that stores a plurality of detection conditions for detecting the target images; the memory comprising: an acquiring unit, executed by the processor, that sequentially acquires the plurality of sequential photographed images on a real-time basis from a photographing unit; a choosing unit, executed by the processor, that chooses a detection condition different from a detection condition of a previous time of a plurality of detection conditions for detecting the target images, the plurality of detection conditions being stored in the condition storing unit; a detecting unit, executed by the processor, that, with the detection condition chosen by the choosing unit, performs a search of an image to detect at least one of the target images from a single photographed image acquired on the real-time basis through the photographing unit by the acquiring unit; and an output unit, executed by the processor, that synthesizes a photographed image sequentially acquired on the real-time basis from the photographing unit and a figure indicating that the target image detected by the detecting unit is contained inside the figure and that sequentially outputs a synthesized image on the real-time basis to an outside of the target image detection device, wherein in a first case where the acquiring unit has acquired a first photographed image from the photographing unit, the detecting unit detects a first target image from the acquired first photographed image with a first detection condition, and the output unit synthesizes a first figure for the detected first target image and either the first photographed image or an a-th photographed image, which is a photographed image acquired subsequently from the photographing unit, and outputs a synthesized image, wherein in a second case where the acquiring unit has acquired a second photographed image from the photographing unit after acquiring the first photographed image, the detecting unit detects a second target image with a second detection condition different from the first detection condition, and the output unit synthesizes the first figure and a second figure for the detected first and second target images and a b-th photographed image that the output unit has acquired from the photographing unit after acquiring the a-th photographed image and outputs a synthesized image, wherein in a third case where the acquiring unit has acquired a third photographed image from the photographing unit after acquiring the second photographed image, the detecting unit detects a third target image with a third detection condition different from the first and second detection conditions, and the output unit synthesizes the first and second figures and a third figure for the detected first to third target images and a c-th photographed image that the output unit has acquired from the photographing unit after acquiring the b-th photographed image and outputs a synthesized image, and wherein the acquiring unit, the detecting unit, and the output unit repeat the first case, the second case, and the third case at a time interval of one to a few frames of the sequentially photographed images until the detecting unit uses all of the plurality of detection conditions necessary to detect all of the target images in the photographed image.