A focus detection device includes: a light-receiving unit that receives a pair of images and outputs a first signal string and a second signal string; a decision-making unit that determines a condition assumed as the light-receiving unit receives the pair of images; a selection unit that selects a correlation operation expression among a plurality of correlation operation expressions in correspondence to the condition; a correlation calculation unit that calculates a correlation quantity between a plurality of first/second signals included in the first/second signal string by applying the correlation operation expression selected; a shift amount detection unit that detects a shift amount between the pair of images; and a focus detection unit that detects a focusing condition. The selection unit selects the correlation operation expression to reduce an extent to which signal intensity difference between the first and the second signal strings affects the correlation quantity.