1. An observation apparatus that observes a sample, comprising: an image acquiring unit that acquires a plurality of images of a plurality of parts of the sample; an image processing unit that receives the plurality of images acquired by the image acquiring unit and processes the received plurality of images; a display unit that displays a plurality of processed images processed by the image processing unit; an input unit for rearranging the plurality of processed images displayed on the display unit in response to an input of a user; and a controller including an indicator-based evaluation value calculating unit that calculates an evaluation value for each of the plurality of processed images in accordance with a plurality of evaluation indicators, wherein: the controller compares an evaluation criterion corresponding to an order of the plurality of processed images rearranged by the input unit with each of the plurality of evaluation indicators to evaluate whether each of the plurality of evaluation indicators is right or wrong, and the controller combines the plurality of evaluation indicators having large correlation coefficients among evaluation criterion corresponding to the order of the plurality of processed images rearranged by the input unit, generates a combined evaluation indicator, calculates a correlation coefficient between the generated combined evaluation indicator and the evaluation criterion, compares, on the basis of the calculated correlation coefficient, the generated combined evaluation indicator with the selected evaluation indicator having the maximum correlation coefficient, and selects an evaluation indicator having a larger correlation coefficient.