The variations of adsorption configurations of pyridine on Ge(100) as a function of exposure amount have been studied using high-resolution photoelectron spectroscopy (HRPES). At low exposure the C 1s and N 1s core levels show a single adsorption state of pyridine with N 1s binding energy at 400.85 eV and C 1s binding energies at 286.7 and 285.9 eV, respectively. Those are attributable to Ge-N dative bonding of pyridine through the lone pair electrons of its N atom, which is consistent with previous STM and theoretical studies. As the molecular exposure increases the N 1s core level reveals not only the dative-bonding state of pyridine but also the appearance of a new chemical state at 398.80 eV, corresponding to the sigma bonding of Ge-N. Such a change of adsorption configuration agrees well with new C 1s features at the low binding energy side, which accounts for the evolution of cycloaddition products with Ge-C bonds. Consequently, di-sigma-bonded cycloadducts coexist with Ge-N dative-bonding products for the final configuration with high density of pyridine on the Ge(100).