PURPOSE: To prevent a false decision due to a change of atmospheric pressure by adjusting a designated range of the intake air quantity according to the atmospheric pressure at the time of deciding the operation of EGR to be at such condition that it can be conducted when the intake air quantity for one stroke of an engine is within a designated range.

CONSTITUTION: Negative pressure from an intake tube negative pressure take-out port 34 is introduced into a diaphragm chamber 27 of a diaphragm device 25 for driving a valve element 24 of an EGR control valve 20 through a temperature sensing valve 32 adapted to open when water temperature is more than a designated value and a negative pressure control valve 30 operated in response to exhaust pressure. A microcomputer 50 to which detection values of an air flow meter 3, an engine speed sensor 56, a water temperature sensor 57, a temperature sensor 59 disposed in an EGR passage 13 and an atomospheric pressure sensor 60 are input is adapted to decide a state where EGR is to be conducted when the intake air quantity for one stroke of an engine is with in a designated range, the upper limit of which is raised according to an increase in atmospheric pressure, and to diagnose the operation of EGR according to a detection value of the temperature sensor 59.