We report the first detection of the pulsed X-ray emission from the fastest millisecond pulsar known PSR B1937+21 (P =1.558msec) with ASCA. The pulsar is detected as a point source above ∼ 1.7keV, with no nebulosity indicated. The source flux in the energy band 2–10keV is found to be f = 4.3 × 10−13erg s−1 cm−2, which corresponds to the luminosity of Lx = 4πD2f ≈ 6.6 × 1032(D/3.6kpc)2 erg s−1, where D is the distance, and correspond to ≈ 6 × 10−4 of the rotation power of the pulsar. The pulsation is found at the period predicted by the radio ephemerides with very narrow primary peaks, the width of which is about 1/16 phase ≈ 100μs near the time resolution limit (61μs) of the observation. The pulsed luminosity within the primary peak (1/16 phase interval) is found to be fp = 4.0 × 10−12erg s−1 cm−2. Although there may be a secondary peak, its statistical significance is too low to identify. Spectra of the whole source region and the primary peak are fitted by power law models to give photon indices of about unity for both.