Processing load on a computer can be reduced while saving the labor of photographing an object to estimate a size of the object. Photographed image acquiring means (101) of a 3D model generating system (1) acquires a photographed image in which a first object and a second object are disposed in a real space. Virtual image generating means (105) generates a virtual image in which a first model representing the first object that is a standard size and a second model representing the second object that is not the standard size are disposed in a virtual space. Parameter determining means determines a size parameter indicating a size of the second object based on a result of comparing the photographed image with the virtual image.