To drastically reduce a drying time and at the same time prevent crack and distortion on drying by heating and softening a lumber, for example, with hot water or steam and drying it while water contained inside is forced to be squeezed out by compression.

A high-temperature (approximately 80-90°C) hot water or steam is supplied into a treatment layer that accommodates a lumber 1 for heating the lumber for approximately 30-120 minutes to soften the lumber. Then, when a piston rod 6a of a push-in cylinder 6 is extended in a draft device 3, a push-in plate 6b advances and forces to push in the lumber 1 from a carry-in port 5a into a draft tube 5. Then, water contained inside is squeezed by successively compressing the lumber 1. Then, a water percentage becomes uniform and a ventilation property inside the lumber 1 improves. Further, a plurality of drying cases 4 accommodating the lumber 1 are pinched between a pair of hot plates 7, are heated by a proper means (for example, a high-frequency heating), and water remaining in the lumber 1 is evaporated.