1. A periphery-monitoring device for a working machine, the periphery-monitoring device comprising: an image processing circuitry configured to generate an output image based on an input image captured by a camera attached to the working machine, wherein pixels of the output image include four output pixels associated with respective values of four input pixels of the input image, wherein, to generate the output image based on the input image, the image processing circuitry is further configured to generate an output interpolation pixel positioned at a coordinate between the four output pixels based on values of only two input pixels specified by coordinates of two respective output pixels of the four output pixels, wherein a straight line connects two arbitrary integer coordinate points of four integer coordinate points where the four output pixels are positioned, the output interpolation pixel positioned at the coordinate between the four output pixels is generated by the image processing circuitry based on the values of the two input pixels specified by the two arbitrary integer coordinate points, and wherein the two input pixels used to generate the output interpolation pixel are selected such that a line segment between the straight line and the coordinate of the output interpolation pixel is the shortest line segment compared to line segments between respective straight lines connecting other two arbitrary integer coordinate points of the four integer coordinate points and the coordinate of the output interpolation pixel.