the invention relates to technology for avoiding the influence of intake air volumetric flow rate changing due to contamination of a throttle valve opening . when the opening of the throttle valve of an internal combustion engine is large , the atmospheric pressure is estimated based on , the intake air volumetric flow rate which is estimated based on the throttle valve opening , and on a separately detected intake air mass flow rate . after this , within a predetermined time for example , when the opening of the throttle valve changes to a small opening while the atmospheric pressure is virtually constant , the atmospheric pressure is similarly estimated based on the estimated intake air volumetric flow rate and on the detected intake air mass flow rate . a relationship between the throttle valve opening and a value related to the throttle valve opening is then learned and corrected so that the latter estimated atmospheric pressure which is influenced by contamination of the throttle valve approaches the former estimated atmospheric pressure of high reliability which is not influenced by contamination of the throttle valve . as a result , the atmospheric pressure can be estimated to good accuracy even when the throttle valve is maintained in the low opening region . in addition , the performance of the various controls based on the throttle valve opening can be improved .