Abstract: It is an object of the present disclosure to provide a method for producing a reduced coenzyme Q10 Form II crystal or a crystalline solid thereof, which is capable of stably producing a reduced coenzyme Q10 Form II crystal or a crystalline solid thereof.

Abstract: It is an object of the present disclosure to provide a method for producing a reduced coenzyme Q10 Form II crystal or a crystalline solid thereof, which is excellent in the filterability of a slurry containing a reduced coenzyme Q10 Form II crystal. The present embodiment is a method for producing a reduced coenzyme Q10 Form II crystal or a crystalline solid thereof, including: adding a reduced coenzyme Q10 Form II crystal as a seed crystal to a mixed solution containing an alcohol and a reduced coenzyme Q10; and precipitating a reduced coenzyme Q10 Form II crystal in the mixed solution after adding the seed crystal, wherein a change rate of formazin turbidity (FTU) is maintained at 15 FTU/min or more for 80% or more of a period during which the formazin turbidity (FTU) shifts from 1,000 to 10,000 in the precipitation.