To apply coating to a steel plate at high treatment efficiency without generating a flaw as much as possible.

The conveying device comprising a projection roller 14 and a foaming roller 15 rotatably supported and supporting the steel plate W from a lower surface side is provided on a coating device 10. The projection roller 14 is made to a structure having a plurality of pins 22 arranged on a peripheral surface of a roller body 21 with a gap in a circumferential direction and having a ball rollably retained at a distal end of the pin 22. The foaming roller 15 is made to a structure having a foaming layer 32 comprising a porous metal material provided on a peripheral surface of the roller body 31 over a circumferential direction. The steel plate W in which front and back surface are coated with a shop primer comprising zinc-rich paint by a coating spray 12 is conveyed by the projection roller 14 and the foaming roller 15.