PURPOSE: To determine a measuring range on a plot in a tissue blood flow rate measuring apparatus using a laser light by a method wherein an incoming point A, a photodetecting point B and a photo detectability limit point C are determined and a positional relationship at points are plotted on a straight line regarded as measuring surface to determine a trace of K obtained.

CONSTITUTION: When an intensity distribution of laser light impinged on a tissue is measured, a spherical intensity distribution is given centered on an incoming point. When the incoming point is on the surface of a tissue, a hemispherical intensity distribution is given centered on the incoming point. This is because a size (μm order) of a cell composing the tissue of an organism is larger than a wavelength (nm order) of the laser light to scatter the laser light, and hence, the intensity at the point C becomes equal to the intensity at the point D. If the point D gives the maximum depth among those determined by a blood current value (wave form), the point C represents a distance of the surface of the tissue maximum determined in blood current. Thus, when a photodetector is positioned between the points A and C, a trace of K obtained by AK+KB=AC can be recognized as measuring range.