As light is allowed to impinge from above a microplate M and an imaging unit 13 (a line sensor 131 and an imaging optical system 132), which moves in a scanning motion along the bottom surface of the microplate M, receives transmitted light, images of wells W formed in the microplate M are captured. The scope of imaging by the line sensor 131 is greater than the diameter of at least one well W, or preferably, encompasses a plurality of wells. When the depth of field of the optical system 132 is 0.6 mm or less, the influence of reflection of side walls of the wells upon the images is reduced.