This paper proposes a two wheels caster type odometer to estimate the current position and orientation of omnidirectional vehicles. The proposed odometer is composed of two passive wheels and one passive rotational axis. The two passive wheels rotate independently, and the passive rotational axis changes the odometer's orientation. Angular velocities of the two wheels and rotational axis are measured by three rotary encoders. In order to solve an encoder's wiring problem by increasing rotation of the rotational axis, a mechanism of a gear train is proposed. Using measured values of the three encoders, the current velocity vector of an omni-directional vehicle is calculated by kinematics of the proposed odometer. Using the velocity vector, the position and orientation of the omni-directional vehicle are estimated by dead-reckoning. An experimental two wheels caster type odometer is developed. Experimental results of localization by the developed odometer are shown in this paper.