An image pickup apparatus that is capable of preventing hunching certainly at low cost and of switching between the day mode and the night mode at an optimal timing. An image sensor outputs an image signal depending on an optical image formed through an image pickup optical system. A mode setting unit sets a photographing mode for photographing using the image sensor from among a day mode and a night mode in which sensitivity for a wavelength range corresponding to infrared light is higher than that in the day mode. An obtaining unit obtains ratio information about the ratios of the infrared light and visible light based on the image signal in the night mode. A condition setting unit sets a determination condition that is used for switching the photographing mode to the day mode from the night mode based on the ratio information.