A crystallization process for forming a stable crystal of N-(3,3-dimethylbutyl)-APM, which comprises using either water or a mixture of water with a lower alcohol as the crystallizing solvent and controlling the crystallization point to 30° C. or above; and another crystallization process therefor, which comprises using either water or a mixture of water with a lower alcohol as the crystallizing solvent and using as the seed crystal a crystal of N-(3,3-dimethylbutyl)-APM exhibiting peculiar peaks of diffracted X-ray at least at diffraction angles (2θ, CuKα ray) of 6.0°, 24.8°, 8.2° and 16.5° to thereby crystallize the above objective crystal preferentially. These crystallization processes enable the constant formation of stable crystals of N-(3,3-dimethylbutyl)-APM at a low cost.