1. An image signal processing apparatus comprising: a basic region providing means for providing a basic region as a basic part to be measured for an input image signal; an increment/decrement value providing means for providing an increment/decrement value in a measurement region; a measurement region determining means for determining the measurement region on the basis of the basic region and the increment/decrement value; a measuring means for measuring in a unit frame period whether each pixel in a designated measurement region of the input image signal has a signal level less than a threshold value; a black band detecting means for detecting in the unit frame period a black band region included in the input image signal based on a measurement result from the measuring means; a calculating means for calculating a scaling ratio of the input image signal while maintaining an aspect ratio thereof, based on a detection result from the black band detecting means; and a scaling means for scaling the input image signal up or down, based on the ratio obtained by the calculating means, wherein the increment/decrement value providing means resets the increment/decrement value to half of the previous value, the measurement region determining means selectively resets a new measurement region by adding or subtracting the new increment/decrement value to or from the previous measurement region depending on whether the boundary between a black band region and an image region is detected by the black band detecting means, and the measuring means performs the measurement on a new measurement region.