A coating nozzle 10 contains an introduction passage 21, an expansion passage 22 and a slit passage 23. The introduction passage 21 introduces high viscosity paint P and the slit passage 23 discharges it. The expansion passage 22 communicating with the introduction passage 21 has a wider space than the introduction passage 21 and substantially sectored-trapezoidal shape. The slit passage 23 is a slit opening communicating with the bottom of the expansion passage 22. The slit passage 23 includes a slit inlet port 23b on the bottom of the expansion passage 22 and a slit outlet port 23a with an arc shape on the other side of the inlet port 23b. This slit passage 23 has Lt/R×100=70-130% wherein the curvature radius of the arc of the outlet port 23a is R and the chord length Lt of the arc of the outlet port 23a is Lt.