It is required to achieve an accurate measurement of 10B distributions and concentrations in the tumor cells for boron neutron capture therapy (BNCT). We employed a technique of neutron capture autoradiography (NCAR) of the whole body samples of tumor bearing mice using CR-39 plastic track detectors to the measurements of 10B accumulations in mice. In order to confirm the validity of the NCAR measurements, the differential REL spectrum for the neutron-induced particles emitted from the standard filter paper sheets was compared with that for 10B-compound solution. Differential REL spectrum for protons originated from the biogenically abundant nuclides through 14N (n, p) 14C and 1H (n, n′) 1H reactions was measured in vivo using a small piece of CR-39 inserted into the tumor of mouse.