PURPOSE: To uniformly and beautifully paint a joint groove, by painting the inside surface of the joint groove by a painting roll as the joint groove is located face- down and by blowing air toward the interior of the joint groove.

CONSTITUTION: When the joint groove 2 of a board 1 is located face-down, a painting roll 3 whose bottom is dipped in a container 4 fitted in the joint groove 2 to paint it. At that time, the excess portion W1 of a paint W coated on the joint groove 2 is slightly moved out onto both the edges 2a of the groove. Air is blown from a nozzle toward the interior of the groove 2 along the edges 2a to push the excess paint W1 into the groove 2 and dry all the coated paint. As a result, the excess paint is not deposited outside and the joint groove is uniformly painted.