1. An optical system for measuring a variation in a concentration of a metabolite or a variation in blood flow in a living body by using a plurality of light irradiators and a plurality of light detectors, the system comprising: a probe to be attached to a head of a subject, the probe having an array of irradiating portions of said light irradiators and light receiving portions of said light detectors, the array including even partial arrays in each of which the irradiating portions and the light receiving portions are alternately arranged in a rhombic lattice configuration obtained by obliquely deforming a tetragonal lattice, the even partial arrays being arranged in a semispherical configuration to compose the array.