In a conventional method (a), high energy ions are generated by irradiation of a focused laser beam onto a micron-thick foil target. In the new method (b), instead of using thin foil targets, the laser beam is focused onto a nanoparticle target. In the case of the nanoparticle target, it is easy to create a state of matter that effectively absorbs laser energy, called subcritical-density plasma. As a result, we demonstrated efficient generation of high energy ions whose energies are ten times higher than those in previous experiments with thin foil.