Proton signals and terahertz (THz) radiation are simultaneously observed in a laser-plasma interaction with a thin-foil target. We also observe the interferogram of the preformed plasma at the same time. The preformed plasma is generated at the font side of the target. Next, to obtain higher energy protons, the size of the preformed plasma is reduced by changing the laser contrast level. In the high-contrast laser pulse case, the maximum energy of the protons generated at the rear side of the target along the target normal direction increases.