Abstract: The treadmill is of the type which comprises: a chassis; an upper structure; a front roller and a rear roller that are horizontally journalled in said chassis, and an endless belt mounted around the front roller and rear roller and presenting an upper path and a lower path. The device comprises two optical sensor assemblies, each comprising an optical emitting means, disposed above or beneath the lower path of said endless belt, and an optical receiving means, disposed in the other of said positions, said optical sensor assemblies operating between a belt aligned condition, interrupting the luminous flux from said optical emitting means to said optical receiving means, and a belt misaligned condition, in which said belt allows the optical flux between one of said optical emitting means and a respective optical receiving means which is operatively associated with a visual and/or sonorous signalizing means to activate said signalizing means upon the misalignment of the belt.