The hydrogen (H) termination of the 2X1 and 3X1 structure and the initial stage of H adsorption on the Si(100) surface are theoretically investigated. For this study, the electronic states induced by the H atoms are calculated in the first-principles local density functional method. The scanning tunneling microscopy (STM) images and the scanning tunneling spectroscopy (STS) spectra are also simulated. The results clarify what the STM depicts on the H-adsorbed Si(100) surfaces. On the termination, two ball-like spots each corresponding to the H atom form a beam shape. At the initial adsorption, the bright spot observed in the images is assigned to the remaining Si atom of the reacted dimers. However, the observed STS spectra of both the surfaces cannot be reproduced. The physical reasoning to amend this discrepancy in the spectra is discussed.