1. An image processing apparatus including a processor and a non-transitory storage medium storing one or more programs of instructions which are executable by the processor to perform and to configure the host computer to comprise a process to recognize a specific region from an image and to configure the apparatus to comprise: a threshold determination unit performing threshold determination for a determination range including a target pixel and multiple pixels surrounding the target pixel among pixels included in the image by applying predetermined threshold conditions to specify the target pixel as a recognized pixel in a case where the threshold conditions are satisfied in any pixel within the determination range; a process execution unit executing the threshold determination repeatedly by moving the determination range sequentially; and a threshold automatic calculation unit that calculates a peak position of a density value distribution histogram of the image, calculates a symmetrical distribution curve that is symmetrical about the peak position using data of a part whose density value is lower than the peak position, subtracts the symmetrical distribution curve from a distribution curve of the histogram to calculate a subtracted distribution curve, and calculates the threshold conditions based on the subtracted distribution curve, wherein the threshold determination unit applies the threshold conditions calculated by the threshold automatic calculation unit to perform the threshold determination.