Disclosed is a field distribution converting optical fiber which can be massproduced and enables mutual conversion elliptical field distribution to circular field distribution or vice versa, and a laser diode module in which the field distribution converting optical fiber is used. The optical axis is assumed to be Z, either of the refractive index profiles of a core in the X and Y directions of the orthogonal three axes X, Y and Z is made into a square-law distribution profile, a slope of the refractive index profile in the corresponding X direction is made different from a slope of the refractive index profile in the Y direction. The optical axis propagating in the XZ and XY planes is caused to cyclically change in the Z direction, wherein these cycles are made different from each other to cause the cross-sectional profile of the field distribution of the optical fiber to be cyclically changed. The optical fiber length is made almost coincident with the length of the common multiple between a one-fourth cycle length of a field distribution change cycle of a light beam propagating in the XZ plane in the Z direction and a one-fourth cycle length of a field distribution change cycle of a light beam propagating in the YZ plane in the Z direction, whereby optical waves having elliptical field distribution, which are made incident into the incident side of the optical fiber are caused to outgo from the outgoing side as circular field distribution optical waves.