A number of radionuclides of interest in nuclear medicine are short lived isotopes that emit only gamma ray. The most of all Dept. of Nuclear Medicine in the hospt. are using the $^{99}Mo-^{99m}Tc$ generator for elution of the short lived isotope $^{99m}TcO_4$. A $^{99}Mo-^{99m}Tc$ generator consists of an alumina column on which $^{99}Mo$ is bound. The parent isotope($^{99}Mo$ : half life 67 hr.) decays to its daughter $^{99m}TcO_4^-$ which is a different element with a shorter half-life. $^{99}Mo$ emitted 41-keV(1.3%), 141-keV(5.6%) 181-keV(6.6%) and 366-keV(1.5%) gamma rays. But $^{99m}TcO_4$ emitted only 140-keV gamma ray. We study about the gamma ray distribution around the $^{99}Mo$ generator. And obtained the result as follows ; 1. Total counted gamma ray from generator smaller in front side than back. 2. The gamma ray emitted from $^{99}Mo$ generator without $^{99m}TcO_4$ vial increased in the back side(Mo column posited side) 3. The gamma ray only from the $^{99m}TcO_4$ vial increased in the front side. 4. Apron can protect gamma ray above 60% of total radiation from the $^{99}Mo$ generator.