  Initial C/O ratios of circumstellar disks have been obtained in solar-type
stars by differential photospheric abundance analysis with respect to the Sun.
We present C/O ratios of M dwarfs obtained by absolute abundance analysis
derived from CO and H2O spectra in the K-band. We compare the distributions of
C/O ratios in M dwarfs and solar-type stars using the Kolmogorov-Smirnov test.
The C/O distribution of M dwarfs is consistent with low frequency of high C/O
ratios in solar-type stars.
