The catalytic deep oxidation of methanol by oxygen on the Pt(111) surface under ambient pressure conditions has been studied by in situ infrared reflection absorption spectroscopy (IRAS). It was found that the Pt(111) surface was covered with formate in a flow of 0.16 kPa of methanol and > 1.3 kPa of oxygen mixture at 350 K while it was covered with CO at the oxygen pressure below 1.3 kPa, CO2 was detected in the product gas and the reaction kinetics was examined. The relation between the production rate of CO2 and the coverage of adsorbates as a function of oxygen pressure indicated that the CO2 mainly originated from the formate which was produced from oxidation of methanol. Although CO produced by methanol decomposition seems to be one of precursors of CO2 formation, this reaction was found to be a minor pathway. The detailed mechanism of oxidation of methanol on Pt(111) was discussed.