1. An image measurement apparatus comprising: an image acquiring unit configured to acquire a sample image that is an image obtained by capturing an image of a sample; a standard color space converting unit configured to convert the sample image into a standard sample image that is an image in a standard color space; a color distribution normalizing unit configured to convert the standard sample image into a normalized sample image by converting pixel values of pixels in the standard sample image to cause a color distribution vector indicating a color distribution of the standard sample image to match a predetermined standard color distribution vector indicating a standard color distribution; and a positive rate acquiring unit configured to detect positive pixels by performing threshold processing on pixel values of pixels in the normalized sample image, and acquire a positive rate indicating a proportion of the detected positive pixels.