A theoretical solution to the free vibration of a string, where both ends are moving and the string length is constant such as a belt, has been already obtained. In this paper, a theoretical solution of forced vibration of an axially moving string, whose length is constant, is presented. Finite difference analyses of string vibration are also performed to verify the validity of this solution and to obtain the vibration characteristics of moving string. The calculated results of the finite difference analyses are in fairly good agreement with those of the theoretical solution. The effect of the moving speed on the resonant frequency and the anti-resonant frequency is clarified in the case that both ends are excited sinusoidally: When both ends are excited in same phase, even an asymmetric mode showed the resonance by the effect of moving.