A linear generator having a fluid pressure cylinder structure, comprising a cylinder having left and right end walls, and a piston reciprocally disposed in the cylinder and having left and right pressure-receiving surfaces such that a left fluid chamber is defined in the cylinder between the left end wall of the cylinder and the left pressure-receiving surface of the piston and a right fluid chamber is defined in the cylinder between the right end wall of the cylinder and the right pressure-receiving surface of the piston, in which a fluidpressure in the left fluid pressure chamber and a fluid pressure in the right fluid pressure chamber are to be alternately applied to the piston in the cylinder to reciprocate the piston in an axial direction, the linear generator further comprising: a permanent magnet band provided between the left pressure-receiving surface of the piston and the right pressure-receiving surface of the piston; and an electromotive coil band formed on a cylindrical wall between the left and right end walls of the cylinder; wherein the piston having the permanent magnet band is arranged to reciprocate in the axial direction to induce power generation in the electromotive coil band; wherein the piston includes a cylindrical yoke and a permanent magnet tube body, said cylindrical yoke comprising a cylindrical portion, and said permanent magnet tube body being supported externally on and disposed radially outwardly of said cylindrical portion of said cylindrical yoke, so that the piston has a cylindrical structure; wherein the permanent magnet band is constituted by said permanent magnet tube body of said piston, said permanent magnet tube body has a hollow tube hole with said cylindrical portion of said cylindrical yoke disposed therein, and said cylindrical portion of said cylindrical yoke has an empty, hollow tube hole, such that said piston has an empty, hollow tube hole open at both ends thereof; wherein said permanent magnet tube body is radially magnetized along an entire length thereof; characterized in that the electromotive coil band is provided over the left and right fluid pressure chambers, and the piston further includes left and right pressure-receiving end plates that define the left and right pressure-receiving surfaces, respectively, and that close the open ends of the hollow tube hole of the piston. a permanent magnet band provided between the left pressure-receiving surface of the piston and the right pressure-receiving surface of the piston; and an electromotive coil band formed on a cylindrical wall between the left and right end walls of the cylinder; wherein the piston having the permanent magnet band is arranged to reciprocate in the axial direction to induce power generation in the electromotive coil band; wherein the piston includes a cylindrical yoke and a permanent magnet tube body, said cylindrical yoke comprising a cylindrical portion, and said permanent magnet tube body being supported externally on and disposed radially outwardly of said cylindrical portion of said cylindrical yoke, so that the piston has a cylindrical structure; wherein the permanent magnet band is constituted by said permanent magnet tube body of said piston, said permanent magnet tube body has a hollow tube hole with said cylindrical portion of said cylindrical yoke disposed therein, and said cylindrical portion of said cylindrical yoke has an empty, hollow tube hole, such that said piston has an empty, hollow tube hole open at both ends thereof; wherein said permanent magnet tube body is radially magnetized along an entire length thereof; characterized in that the electromotive coil band is provided over the left and right fluid pressure chambers, and the piston further includes left and right pressure-receiving end plates that define the left and right pressure-receiving surfaces, respectively, and that close the open ends of the hollow tube hole of the piston.