A boundary line detection apparatus and method and an image processing apparatus and method as well as a recording medium by which the jaggy in the proximity of a boundary line is removed without increasing the sampling rate to remove are disclosed. The differences between pixel values between a noticed pixel and pixels in the proximity of the noticed pixel are compared with each other to detect whether or not a boundary is present in the proximity of the noticed pixel. Then, a length, a direction, and a start point and an end portion of the boundary line are determined. Then, the distance between the noticed pixel and the boundary line is calculated, and the pixel value of the noticed pixel is weighted using the distance. Then, the pixel obtained by the weighting is outputted.