The large volume of fluid mud deposits with softsediment properties like silty sand or clay in the approach channels has become the significant obstacles for keeping sufficant water depth. In these circumfluences, it is eager to demand to develope new measuring system for the mud density at the seabottom in order to grasp the situations of fluid mud existing on regular sea bottom.
　In this study, the developement of mud layar density meter with gamma ray is described, and theoretical or experimental discussion on the relationship between gamma ray counts and mud density are also treated. From the resluts of the study, it is revealed that the use with gamma ray absorption should have advantages on measuring stability compared to theuse with gamma ray scattering. Finally, in the study, the layer density meter for practical use wasdeveloped on the basis of gamma ray absorption system, which brought another experimental results on its measurring characteristics, accuracy, etc.against provided pseudo fluid mud.