PURPOSE: To reduce an amount of water drop in heating of a sheet by installing single cross-directionally oriented outlet on the bottom wall of a steambox jetting a condensable gas and installing a dam member on the inside of the outlet to inhibit dropping of the condensate.

CONSTITUTION: A steam room 32 and vacuum aspirating room 52 are formed on the inside wall part of the steambox through a heat insulating material 40 and separated by the heat insulating partition 62. An outlet 46 and an inlet 54 are formed on a bottom wall 24 cross-directionally oriented to the traveling direction of a sheet 8, and forming dam parts 64 on the inside edges of the outlet and inlet and setting to prevent drop from the outlet and inlet even when the steam condensates and accumulating on the bottom of the device.