1. A method of calculating a photon path distribution inside a scattering medium, comprising: a first step of dividing a measured medium being the scattering medium, into N (1&lE;N) voxels each having a predetermined size and assumed to have a uniform absorption distribution in a measurement wavelength region; a second step of hypothesizing an imaginary medium that can be assumed to have the same shape, boundary conditions, and scattering distribution as the measured medium, have a uniform refractive index distribution, and be nonabsorbing, for the measured medium divided into the N voxels; a third step of setting a light injection position for injection of impulse light, and a light detection position for detection of an impulse response s(t) at a time t of the impulse light having been injected at the light injection position and having propagated in the imaginary medium, on a surface of the imaginary medium; a fourth step of calculating the impulse response s(t) detected at the light detection position after injection of the impulse light at the light injection position into the imaginary medium; a fifth step of calculating a photon existence probability density Ui(t′,t) that an ensemble of photons constituting the impulse response s(t) detected at the time t at the light detection position after injection of the impulse light at the light injection position into the imaginary medium has existed in an arbitrary voxel i (i&equals;1, 2,..., N; 1&lE;N) in the imaginary medium at a time t′ (0&lE;t′&lE;t) before the detection at the light detection position; a sixth step of calculating a photon existence cumulative probability Ui(t) that the ensemble of photons constituting the impulse response s(t) detected at the time t at the light detection position has existed in said voxel i, using the photon existence probability density Ui(t′,t); and an eighth step of calculating a time-resolved path length li in the voxel i, using the photon existence cumulative probability Ui(t) and the impulse response s(t).