We have investigated the martensitic transformation behavior in a single crystal of Ni2MnGa under various magnetic field. The single crystal used in the present study exhibits an intermediate phase (I-phase) transformation at TI = 250 K and a martensitic transformation at TM = 202 K. Since the martensite phase (M-phase) of Ni2MnGa has a large magnetocrystalline anisotropy, the effect of magnetic field depends significantly on the direction of magnetic field. We have measured the reverse (i.e., M-phase to I-phase) transformation start temperature As from a single variant state to examine the effect of magnetic field because the forward (I-phase to M-phase) transformation usually forms a multivariant state of the M-phase. When the magnetic field is applied parallel to the easy axis, As increases linearly with increasing magnetic field. On the other hand, when the magnetic field direction is not parallel to the easy axis, As decreases in a low field region and then increases on further increasing the magnetic field. Such behavior of magnetic field dependencies of As are quantitatively explained by the Clausius-Clapeyron equation, where we have assumed that the magnetic field dependence of As agrees with the magnetic field dependence of the equilibrium temperature.