1. An automatic follow-up device for a camera, comprising: detecting means for detecting a moving direction of a heat source which is an object located within a field of a camera as an object to be photographed; follow-up means arranged to cause a photo-taking optical system to follow said heat source by changing a direction of an optical axis thereof according to a result of detection made by said detecting means; distance measuring means for detecting a distance to said object; and control means arranged to have a magnification value of said photo-taking optical system changed according to the distance detected by said distance measuring means to a magnification value at which an image of said object being followed up can be obtained on an image plane of the camera approximately in a constant size.