  The adequate interpretation of scanning tunneling microscopy (STM) images of
the clean Si(001) surface is presented. We have performed both STM observations
and {\it ab initio} simulations of STM images for buckled dimers on the clean
Si(001) surface. By comparing experimental results with theoretical ones, it is
revealed that STM images depend on the sample bias and the tip-sample
separation. This enables us to elucidate the relationship between the
corrugation in STM images and the atomic structure of buckled dimers. Moreover,
to elucidate these changes, we analyze details of the spatial distributions of
the $\pi$, $\pi^{\ast}$ surface states and $\sigma$, $\sigma^{\ast}$ Si-Si bond
states in the local density of states which contribute to STM images.
