1. A diagnostic support apparatus comprising: a hardware processor; and a non-transitory memory having stored thereon executable instructions, which when executed by the hardware processor, cause the diagnostic support apparatus to operate as: a base vector matching unit configured to match test image base vectors and normal image base vectors, the test image base vectors being different from the normal image base vectors, the test image base vectors being used as a basis to represent a test feature quantity that is an image feature quantity of a test image in which presence of an image of a lesion site is unknown, and the normal image base vectors being used as a basis to represent a normal feature quantity that is an image feature quantity of a normal image that does not include an image of a lesion site; a lesion determination unit configured to determine that the test image includes an image of a lesion site when a difference between a test image base coefficient and a normal image base coefficient is greater than a determination threshold value, the test image base coefficient being a coefficient with which the test feature quantity is transformed to a base representation, and the normal image base coefficient being a coefficient with which the normal feature quantity is transformed to a base representation; and a determination result output unit configured to output a result of the determination by the lesion determination unit, wherein the base vector matching unit includes: a pixel value transformation unit configured to, when the test feature quantity and the normal feature quantity do not match in type, transform the test feature quantity to a pixel value to restore the test image prior to transformation of the test image to obtain the test feature quantity, the pixel value being obtained by using an inverse of the transformation used to obtain the test feature quantity; an image feature quantity calculation unit configured to calculate an image feature quantity of the same type as the normal feature quantity from the restored test image; and a base representation unit configured to represent the image feature quantity calculated by the image feature quantity calculation unit as a linear combination of the normal image base vectors with a coefficient, and the lesion determination unit is configured to determine that the test image includes an image of a lesion site when a difference between the normal image base coefficient and the coefficient used by the base representation unit to represent the calculated image feature quantity is greater than the determination threshold value.