To keep the coke grain size appropriate for improvement on coke oven productivity without adding a carbon material or pitch to the coal.

In the carbonization of coal in a coke oven for the preparation of coke, a total of 50 cm2-1,000 m2 per 1 m3 of carbonization chamber volume of sheets made of a material comprising pulp or wood not to soften during carbonization are mingled into the coal before carbonization, each sheet as large as 20 cm2-1,000 cm2.