The adsorption state of 1,4-cyclohexadiene on a Si(100)(2x1) surface is studied with synchrotron radiation photoelectron spectroscopy (PES), high-resolution electron energy loss spectroscopy (HREELS), and scanning tunneling microscopy (STM). The existence of one st bond in the chemisorbed 1,4-cyclohexadiene is confirmed by valence band PES and HREELS measurements. The bonds between the 1,4-cyclohexadiene molecule and Si surface are characterized by the observation of a Si-C stretching mode in HREEL spectra and the interface component due to the Si-C bond in high-resolution Si 2p spectra. The remaining outermost pi states in the adsorbed molecule are observed directly in the STM images. From HREELS and PES results, it is also clarified that the chemisorption state is basically invariable at any coverage.