PURPOSE: To shorten drying time and prevent cracking upon drying by a method wherein lumbers are heated and softened in a high-temperature high-pressure vessel to squeeze the moisture of the lumbers out by compressing them and, thereafter, the lumbers are restored to a condition of slightly compressed whereby the lumbers are dried.

CONSTITUTION: When lumbers 1 are carried into a high-temperature high-pressure vessel 2 to receive them into a receiving mold 10 of a press 3 and a lid 5 is closed to seal the vessel 2, then, high-temperature high-pressure steam is supplied into the vessel 2 through a steam pipeline 6 to heat the lumbers 1, the lumbers 1 absorb the steam and are softened. When the plunger 12 of the press 3 is descended to compress the lumbers 1, moisture in the lumbers is squeezed out and the moisture content of the lumbers l is unified. When the compressing condition is released to bring the lumbers 1 into a condition that a pressure is applied on them slightly and the steam in the vessel 2 is discharged through an exhaust pipeline 8 to reduce the pressure in the vessel, the lumbers are brought into dried condition. The moisture, squeezed out of the lumbers 1, is recovered through a steam drain pipeline 7, then, high-frequency electric current is supplied to the receiving mold 10 and the plunger 12 to dry the lumbers through high-frequency heating whereby the drying of the lumbers 1 is finished.