A submillimeter heterodyne receiver with an open structure mixer and a laser local oscillator was used to detect the CO (J=6←5) absorption transition at 691.473 GHz in the earth's upper atmosphere. Spectra were taken toward the sun with a 1024-channel acousto-optical spectrometer. The resolution was 250 kHz. The CO volume mixing ratio profile was calculated from the spectral line shape. It is shown that the CO (J=6←5) measurement results in a well-determined CO profile for the lower mesospheric layers, as compared with the results of other for observations of CO transitions with lower J values.