It has been noticed, that when a obstacle moves through a fluid, it seems as if the mass of the obstacle being increased by some action of the fluid, and such effect has been called as“Virtual mass”or“Additional mass”. But there is many kinds of definitions about so-called“Virtual mass”. It can be defined by the difference of some physical amounts containing the mass-such as inertia force, kinetic energy, and period of oscillations-between in the vacuum and in a fluid. The Author showed in this paper, that these virtual masses defined by several different physical amounts are not always the same, and consequently some attentions must be taken at adopting a value of a virtual mass, defined by a physical amount to another physical amount, and also made clear the relations between them.