In image-checking equipment, a detector reads a line-shaped check image formed on a sheet and acquires image data on the check image. A controller calculates an edge blur in a rising edge and a falling edge of the image data, calculates a line width of the check image. The controller refers to a correction table with the measured values of the edge blur and the line width of the check image, and acquires a corrected line width value based on the correction table to obtain the real line width of the check image.