1. A user detecting apparatus comprising: a memory that stores a program including a procedure; and a processor that executes the program, the procedure comprising: obtaining a first image captured by a camera and a second image captured after the first image has been captured, extracting a user-associated area, which is associated with a user, from the first image according to a given condition, dividing the user-associated area into a plurality of areas, storing a histogram of each of the plurality of areas in the memory, detecting from the second image a corresponding area that corresponds to an area that is one of the plurality of areas and has a first reference histogram, which is one of the color histograms of the plurality of areas, according to similarity between the first reference histogram and a color histogram involved in the second image, and changing a reference histogram used for a third image, which is captured after the second image is captured, from the first reference histogram to a second reference histogram.