The present invention provides a digital camera which has a plurality of image capturing optical systems and quickly performs automatic focusing. A digital camera includes a first image capturing optical system formed from a lens 2 and a first image sensor, and a second image capturing optical system formed from a lens 3 and a second image sensor. An image processor detects a phase difference between an image of a single subject formed on the first image sensor and an image of the same formed on the second image sensor, in order to perform phase-difference AF. Since the lenses 2, 3 have different focal lengths, the image processor executes correlation operation after having performed conversion processing for causing a scaling factor of the image formed by the first image sensor to coincide with that of the image formed by the second image sensor, to thus detect a phase difference.