1. An in-vehicle target detecting device comprising: image acquiring means for acquiring, at a predetermined measurement cycle, a captured image which is an image capturing an area ahead of an own vehicle; positional information acquiring means for transmitting and receiving radio waves to acquire positional information indicating at least an orientation and a distance of at least one target candidate in relation to the own vehicle, the target candidate reflecting the radio waves; image recognition means for performing image recognition to detect a detection object by searching a predetermined image search area in the captured image, the recognition means storing at least an image detection position which is a position of the detection object in the captured image; search area setting means for setting the image search area based on an image-plane measurement position which is a position corresponding to a measurement position in the captured image, the measurement position being a position in three-dimensional space of the target candidate indicated by the positional information acquired by the positional information acquiring means at a timing at which the captured image is acquired by the image acquiring means; and target state estimating means estimates whether or not the target candidate is configured by a plurality of detection objects, wherein the search area setting means includes: base search area setting means for setting a predetermined first search area set as the image search area, when the target state estimating means estimates that the target candidate is not configured by the plurality of detection objects; and search area switching means for switching the image search area from the first search area to a second search area which is an area obtained by broadening the first search area in a lateral direction matching a vehicle width direction of the own vehicle in the captured image, when the target state estimating means estimates that the target candidate is configured by the plurality of detection objects.