1. A smoke detecting apparatus for detecting occurrence of smoke by subjecting an image captured by a monitoring camera to image processing, the smoke detecting apparatus comprising: an image memory for storing a plurality of images captured by the monitoring camera in a time series; and a smoke detection area selecting portion for calculating a luminance histogram of the same pixel for each predetermined pixel a plurality of times in a past predetermined period based on the plurality of images stored in the image memory, detecting presence or absence of a luminance value that has been newly generated due to occurrence of one of an intrusive object and smoke based on the luminance histogram, and identifying candidate regions to be subjected to the image processing.