1. An image pickup apparatus comprising: an image pickup unit that converts an optical image of an object received via an imaging lens into an image signal; an exposure control unit capable of controlling an exposure time of the image pickup unit such that the object image is underexposed relative to an appropriate exposure; an image processing unit that generates a display image signal by implementing predetermined dynamic range expansion processing on an image signal obtained by performing underexposed image pickup; a warning region detection unit that detects an image region in which a pixel within a predetermined gradation range exists as a warning region from the image signal either prior to implementation of the predetermined dynamic range expansion processing or following implementation of the predetermined dynamic range expansion processing; a synthesis unit that synthesizes the display image signal generated by the image processing unit with a warning pattern corresponding to the warning region detected by the warning region detection unit; and a display unit that displays the image synthesized by the synthesis unit as a live view image, wherein the image processing unit performs the predetermined dynamic range expansion processing by performing different gradation conversion processing on each pixel of the image, and the warning region detection unit detects a region in which a loss of gradation occurs in accordance with a dynamic range expansion ratio as the warning region in relation to the image signal prior to implementation of the predetermined dynamic range expansion processing.