An optical branch device, comprising: a branch portion having a first branch waveguide extended linearly and continuously from a waveguide in a basis portion, and second and third branch waveguides extended in the directions farther from the first branch waveguide in such a manner that the first branch waveguide is interposed between the second and third branch waveguides; a parallel portion in which the second and third branch waveguides are positioned in parallel to the first branch waveguide; and a divergent portion in which the second and third branch waveguides are extended gradually farther from the first branch waveguide; wherein the interval pitch P (.mu.m), in the transverse direction, between the first, second and third branch waveguides in the parallel portion and the refractive index difference (.DELTA.n) between the core and the clad are set in such a manner that C satisfies a relationship of C=0.5.+-.0.2 wherein C is calculated by substituting P and .DELTA.n for an equation of C=.DELTA.n.times.P.sup.2.