A single crystal is produced by contacting polycrystal showing a discontinuous crystal grain growth at a temperature not lower than T DEG C to a seed single crystal having substantially the same crystal structure as the polycrystal and heating the contacted crystals at a temperature (t DEG C) lower than the temperature T DEG C to cause a solid phase reaction at an interface between microcrystal grains composing the polycrystal and the seed single crystal, whereby the microcrystal grains in the polycrystal are integrated to the seed single crystal to grow the single crystal.
