An image processing apparatus extracts a contour pixel of a binary input image signal in a contour pixel extraction portion, and calculates a line thinning amount from the extracted contour pixel based on a preset parameter in a line thinning amount calculation portion. The apparatus judges whether black pixel areas of the binary input image signal are connected in a finite area including a target pixel in a finite area connectivity judgment portion. The apparatus adds a line thinning amount to the binary input image signal of the target pixel in accordance with the judgment by the finite area connectivity judgment portion and outputs obtained results in a selective correction output portion.