The dependences of the observed maximum proton energy on laser fluence, and the type and thickness of the target material were studied. The maximum proton energy depended on the laser fluence rather than on the laser intensity for laser pulses with energies between 40 and 430 mJ and pulse durations between 55 and 400 fs. The maximum proton energy increased as the product of target thickness and target mass density decreased. When a polyimide tape target of 7.5 μm thickness was irradiated, the observed maximum proton energy and flux, counted as the number of etched tracks with a solid angle of 0.25 sr, were 3.3±0.1 MeV and 4 × 10⁶/MeV/shot, respectively. For efficient proton acceleration when a wide ion emission angle is acceptable, polyimide was suitable for the tape target because it contains much hydrogen, and furthermore, it has a low mass density and a high tensile strength.