PURPOSE: To reduce the width of variation area measured value with changes of illuminating conditions, etc., by increasing the measurement precision of the area measured value of an object area

CONSTITUTION: A light and shade image is binarized by a binarizing circuit 10 into a binary image and a pixel counter 11 counts pixels in the object area of the binary image. An edge detection part 12 detects the edge of the binary image and a correction value measurement part 13 calculates an area correction value from the density values of pixels before and behind the edge and a binarization threshold value in response to the edge detecting operation. A line quantity measurement part 14 counts horizontal scanning lines including the object area. A CPU 19 subtracts the counted value of the line quantity measurement part 14 from the counted value of the pixel counter 11 and adds the area correction value to the value to calculate the area.