A signal processing device (500a) subjects an input signal indicating an image to a process of sharpening the image and outputs a signal indicating the sharpened image, and includes: a two-dimensional filter (200) for removing, from the input signal, a high frequency component of frequency components in directions other than horizontal/vertical directions of the image indicated by the input signal, to generate an obliquely reduced signal; a sharpening process section (100A) for generating a sharpened signal (S101) by a nonlinear process section (102) performing a nonlinear process on a signal indicating pixels adjacently aligned in a horizontal direction of an image indicated by the obliquely reduced signal; and a sharpening process section (100B) for generating a sharpened signal (S102) by a nonlinear process section (102) performing a nonlinear process on a signal indicating pixels adjacently aligned in a vertical direction of an image indicated by the sharpened signal (S101).