An effort to determine the spatial location of the main source of short-period comet nuclei is made. Some numerical calculations for the orbit evolution of Jupiter’s family comets, comets with middle-period orbits, and Centaur’s group bodies are carried out. The calculations show that orbit evolutions for small bodies of the solar system proceed mainly in the direction of an increase of semi-major axes. This relates to the bodies which can undergo encounters with planets and orbit evolutions of which occur mainly due to gravitational perturbations. The conclusion is made that there is good reason to search the main source of the nuclei for Jupiter’s family comets at distances not more than 6 AU from the Sun.