PURPOSE: To diagnose a fuel pressure control unit solenoid switching reference pressure for a pressure regulator.

CONSTITUTION: In an air-fuel ratio feedback control region, the reference pressure of a pressure regulator 9 is switched over to atmospheric pressure by forcible ON control. The average value α1 of an air-fuel ratio feed-back correction factor α under the aforesaid control condition is computed. Subsequently, reference pressure is switched over to engine intake negative pressure by executing OFF control S9 on the solenoid 13. And the average value α2 of the air-fuel ratio feedback correction factor α is computed. In this place, when the absolute value α3S11 of deviation between the average values α1 and #2 is found to have been less than a specified value αs, the solenoid 13 is judged that it has failed.