In order to reasonably supervise internal exposure of radiation workers, a calculation program that strictly presumes intake of radioactive substances using their concentration and thereby appropriately evaluates an extent of internal exposure has been originally developed. Advanced evaluation using the program is characterized by introduction of two factors, one is the derivative function, that reflects a gradual decrease in the concentration of radioactive substances caused by a ventilation ability of the concerned laboratory, and the other is to give careful consideration to the differences between times at which radiation workers go in (T₁) and out of the lab and times at which air sampling is initiated (T_CA) and terminated. Intake of radioactive substances evaluated by the previous method is underestimated in case T_CA is larger than T₁, whereas it is overestimated in case T_CA is smaller than T₁. Furthermore, it is much more amplified as an interval between T_CA and T₁ grows larger. In both cases, it was found that differences between the intakes evaluated by the previous and current methods become much larger as the effective displacement volume grows larger or the volume of the concerned lab grows smaller.