We confirmed the coverage dependent variation of tautomers of 2-mercaptothiazoline (the thiolate and thione forms) adsorbed on the Ge(100) surface under UHV conditions by using HRXPS measurements in conjunction with the DFT calculation method, which was studied before only in aqueous systems. The C 1s, S 2p, and N 1s core-level spectra obtained using HRXPS revealed the simultaneous presence of two distinct adsorption structures in different proportions at both low (0.15 ML) and high (0.65 ML) coverages. Moreover, we modelled the adsorption structures and geometric configurations of the bond states of 2-mercaptothiazoline on the Ge(100) surface by using the DFT calculation method, and found that the S dative bonded structure is the most stable adsorption structure for the thione form of 2-mercaptothiazoline and that the S-H dissociated-N dative bonded structure is the most stable adsorption structure for the thiolate form.