A measuring apparatus was devised to determine the dynamic physical properties of a powder bed from the transfer function of vibration acceleration that results from the direct application of random vibration to the powder bed. By using this apparatus, the variation of the stiffness constant of a powder bed with various vibration forces and compression forces was measured. The stiffness constant gradually decreased when the vibration force exceeded a certain value. With respect to the change in stiffness constant with compression force, the following experimental equation was derived : k=SW(n), where k is the stiffness constant of a powder bed, W the compression force applied to the powder bed, and S and n are empirical constants.