1. An image processor that processes image data obtained as a result of image pickup by an image pickup device to form processed image data, comprising: a synthesis unit configured to synthesize a plurality of image data items to form synthesized image data; a setting unit configured to set a defect reference level for use in determining whether or not each of pixel data items forming the plurality of image data items is a defective pixel data item, and determining whether or not each of synthesized pixel data items forming the synthesized image data is a defective pixel data item; a defect determination unit configured to determine whether or not the each of the pixel data items is a defective pixel data item and whether or not the each of the synthesized pixel data items is a defective pixel data item, according to the set defect reference level; and a controlling unit configured to control the defect determination unit to compare a pixel data item-pixel value, which is indicated by the each of the pixel data items in the each of the plurality of image data items, with a first defect reference level set by the setting unit, and to control the defect determination unit to compare a synthesized pixel data item-pixel value, which is indicated by the each of synthesized pixel data items in the synthesized image data, with a second defect reference level, different from the first defect reference level, set by the setting unit.