1. A learning apparatus comprising: a weak discriminator generation unit which generates a weak discriminator which calculates a discrimination score that shows whether a target instance of is a positive instance based on the feature extracted from a plurality of bags and a bag label which is information whether each bag is a positive bag or a negative bag, a weak discrimination unit which calculates the discrimination score based on the weak discriminator generated by the weak discriminator generation unit, an instance probability calculation unit which calculates a probability (instance probability) that the target instance is an instance (positive instance) of the correct target object based on the discrimination score calculated by the weak discrimination unit, a bag probability calculation unit which calculates a probability (bag probability) that no smaller than two positive instances are included in the bag based on an instance probability calculated by the instance probability calculation unit, and a likelihood calculation unit which calculates the likelihood which expresses plausibility of the calculated bag probability in the bag probability calculation unit based on the bag label.