Black widow pulsars are millisecond pulsars in a binary system with white dwarfs of mass less than $sim 0.1$ solar mass. It is thought that the ultra-relativistic pulsar wind blows off the atmosphere of the companion, which eventually evaporates away. The acceleration process of the pulsar wind is not well established since there has been no direct observation of emission from the pulsar wind. By using the Fermi Large Area Telescope, we have found evidence on detection of $gamma$-ray emission of the pulsar wind from the Black Widow Pulsar PSR B1957+20, which is modulated with the orbital phase of the binary at energies above $sim 2.7$ GeV. It is suggested that the orbital phase-dependent $gamma$-ray emission is produced in the intra-binary region via the inverse-Compton process.