We observe the proton signals produced by laser interaction with thin-foil targets of polyimide and of copper. We change the thickness of the polyimide target to $7.5{mu}m$, $12.5{mu}m$, and $50{mu}m$. High-energy protons with the maximum energy of ${sim}2.3;MeV$ from $7.5{mu}m$ thick polyimide are observed. This proton beam with the maximum energy of multi-MeV has various applications such as a proton shadowgraphy.