A high-silicon steel strip is manufactured. A basic configuration includes partition plates arranged in the longitudinal direction of a furnace to extend from a position in the vicinity of respective gas nozzles to be in parallel to the pass line of the steel strip, and obstacles arranged to face partition-plate rear edges in the longitudinal direction of the furnace to obstruct the flow of the gas along the steel strip so that siliconizing spaces surrounded by the steel strip, the partition plates, and the obstacles are formed; and gaps between the partition-plate rear edges and the obstacles and so forth which form exhaust passages through which gas is discharged from the siliconizing spaces to other spaces inside the furnace so that treatment gas which has been sprayed from the gas nozzles onto a surface of the steel strip to flow through the siliconizing spaces is discharged through the exhaust passages.