The dosimetry was performed in order to evaluate accuracy of an approximation formula of dose rate in multi-layer of rice separated with thin iron cylinders. Main results are: (1) Energy absorption build-up factor of multi-layer of rice was independent of radius of the multi-layer, radius and thickness of the iron cylinder and an incident angle of gamma-ray to the multi-layer. (2) Approximation formula of the build-up factor of the multi-layer was consisted with Broder's approximation formula of the build-up factor in multi-layer and previously introduced formula of the build-up factor in infinite medium. (3) Derived value of the build-up factor from dosimetry agreed with approximated values within +/- 3%. Standard deviation of the derive values was 7%. (4) In the irradiation regions of a pilot scale irradiator, approximated values of dose rate agreed with the measured values within +/- 8.5%. (5) Approximated values of rice passed through the region agreed with the measured values within +/-5.7%.