To provide a method for forming a large-size organic single crystal excellent in crystallinity and to provide the organic single crystal.

Gel 3 is formed by charging a gelling agent 2 into a solution 1 in which organic matter constituting the organic single crystal 4A is dissolved. The organic matter is supersaturated by cooling the gel 3 and then the crystal nucleus 4 of the organic matter is generated in the gel 3. In a method that the organic single crystal 4A is formed by growing the crystal from the crystal nucleus 4 formed in the gel 3, the optimal property of the gel 3 for crystal growth is specified. It is favorable that complex viscosity by gel oscillation is adjusted to 0.2 Pa s or more and 600 Pa s or less at 25°C and at a frequency of 1 Hz.

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