Using high-resolution microbeam X-ray diffraction, we investigated in-plane anisotropic strain distributions within InGaN/GaN multiple quantum well structures on an m-plane GaN substrate. With this strain analysis, the micro-reciprocal space map (micro-RSM) and transmission electron microscopy measurements supported a structure without any dislocations and strain relaxations. From examining the microarea two-dimensional intensity profile map of the InGaN reflection peaks in the micro-RSMs, we consider that the in-plane anisotropic strain in the m-plane inclines toward the [1120] direction rather than the [0001] direction. On the basis of the slip system in the m-plane via the {1010} prism plane with <1120>-type slip directions that we have advocated, this anisotropy is in agreement with the causes of the strain relaxation in the prismatic plane with slip systems in the m-plane.