To obtain a coating film having uniform thickness even if the cross-sectional area of a coater (an agent coating part) is smaller than that of the conventional.

This device is composed of an agent coating part 1 and an agent supply means for supplying the agent to an agent holding chamber 5 of the agent coating part 1. In the agent coating part 1, the agent holding chamber 5 for holding an agent and an agent introducing passage 6 continued from the agent holding chamber 5 are formed inside and, in the end part facing a supporting body to be coated with the agent, an agent discharge port 7 continued from the agent introducing passage 6 is formed along the coating width direction of the agent. In such a case, the distance D between the center 10 of the agent holding chamber 5 and the agent discharge port 7 is controlled to 1/5 to 4/5 of a dimension L along the direction almost vertical to the surface of the supporting body at the time of applying the agent in the agent coating part 1 the length T of the agent introducing passage 6 is controlled to 1/8 to 5/8 of the dimension L along the direction almost vertical to the surface of the supporting body at the time of applying the agent in the agent coating part 1.