The typical spectrum of the prompt emission of gamma-ray bursts (GRBs) indicates that the electron cooling is suppressed in spite of the strong magnetic field in the standard synchrotron model. Recent Particle-in-Cell simulations show that the particle acceleration by magnetic reconnection in a magnetically dominated plasma can lead to small pitch angles especially in low-energy region. Such a small pitch angle prevents electrons from cooling via synchrotron radiation. In this paper, taking

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... account the effects of the synchrotron cooling and the adiabatic cooling, we numerically calculate the synchrotron spectra with anisotropic electron distributions. If we require a Poynting flux larger than $10^{50}~\mbox{erg}~\mbox{s}^{-1}$ as the model is motivated by magnetic reconnection, the bulk Lorentz factor of $\sim 1000$ and the electron minimum Lorentz factor of $γ_{\rm min}\sim 10^4$ are required to reproduce the typical GRB spectrum.