PURPOSE: To measure a mode power distribution easily and exactly, by measuring the optical transmittivity of a connecting point of a fiber to be mesured and a fiber having the same structure parameter as said fiber, as a function of an axial shift, and calculating the mode power distribution.

CONSTITUTION: When a focusing type multi-mode fiber to be measured 4 and a focusing type multi-mode fiber for measurement 5 which are held in a V-groove of an optical fiber supporting base, respectively, and have an almost same structure parameter are connected, and the supporting base of the fiber 5 is shifted from the supporting base of the fiber 4, an axial shift between the fiber 4 and 5 is varied. When a transmittivity T(S) of the connecting point of the fibers 4, 5 is derived by measuring and deciding the ratio S to the radius of a core of this axial shift, and measuring a light source beam from a light source 1 passing through an optical system 3 and the fibers 4, 5 to the ratio S, by an optical power meter 6, an optical power distribution P(u) to each mode is operated and calculated by an operation based on the expressionI, and a mode power distribution is measured easily and exactly. In the expression, (m) and M denote a mode number and a cut-off mode number, respectively.