1. An image processing device comprising: a pixel extracting unit that extracts pixels serving as a candidate for a nucleus from pixels included in a captured image obtained by imaging a sample including a target cell having the nucleus; a connected-pixel-group extracting unit that extracts a connected pixel group constituted of a predetermined number of adjacent connected pixels or more from the pixels extracted by the pixel extracting unit; and a determining unit that determines a value indicating a possibility that the target cell is included within a region surrounding the connected pixel group extracted by the connected-pixel-group extracting unit, the value indicating the possibility being determined based on a value obtained by aggregating image feature amounts expressing luminance gradients determined based on luminance distribution with respect to a blue component within a plurality of different partial regions included in the region surrounding the connected pixel group extracted by the connected-pixel-group extracting unit without performing normalization on each partial region, the value indicating the possibility being determined also based on a condition for an image feature amount machine-learned based on a value obtained by aggregating image feature amounts expressing luminance gradients determined based on luminance distribution with respect to a blue component within partial regions included in a region occupied by a sample image without performing normalization on each partial region.