To accurately diagnose a leakage in an evaporated fuel-processing apparatus without inviting a large cost increase.

The air from an air pump set so as to supply a secondary air to a discharge path is introduced into a closed process route of an evaporated fuel-processing apparatus at the operation start time at low temperatures for supplying the secondary air. The process route is consequently pressured (S1-S6). The area of a leaking hole is estimated based on a pressure decrease characteristic after the pressuring is stopped (S10, S11) and a volume of the process route estimated based on the pressuring time and the reached pressure.