The realization of a compact accelerator based on laser-driven plasma acceleration is expected owing to the high acceleration field which is 1000 times higher than that of conventional accelerators. So far, a monoenergetic electron beam has not been obtained in laser-driven plasma acceleration. We generated a monoenergetic electron beam for the first time. A dense gas jet was irradiated by a 100-mJ, 50-fs laser pulse with the wavelength of 800 nm. The monoenergetic electron beam with the energy of 7 MeV has been obtained in the acceleration length of 0.5 mm from the plasma with the electron density of 1020 cm-3. (Figure) The monoenergetic electron beam generation is a breakthrough in realizing a compact laser-driven plasma accelerator.