1. A learning device comprising: a feature-quantity extraction circuit that extracts a feature quantity from a feature point of each learning image of a plurality of learning images, the plurality of learning images including a first learning image including a detection target and a second learning image not including the detection target; an acquisition circuit that acquires, from an external device, a transfer classifier for detecting the detection target; a weak-classification calculation circuit that calculates a classification result of the detection target according to a weak classifier for each learning image by substituting the feature quantity extracted by the feature-quantity extraction unit that corresponds to the weak classifier into the weak classifier with respect to each of a plurality of weak classifiers constituting the transfer classifier; and a classifier generation circuit that generates a classifier using the transfer classifier and the weak classifier selected from the plurality of weak classifiers based on the classification result of each of the plurality of weak classifiers.