To provide an image forming device in which a transfer belt is firmly held, the accuracy of the end surface thereof is held, the durability thereof is enhanced, the meandering thereof is eliminated and a high-quality image without color slippage is obtained.

This device is provided with a photoreceptor drum 3 on which a toner image is formed, a transfer belt 11 traveling by being brought into roll-contact with the drum 3 and carrying a sheet of paper P, a transfer roller 23 pressing the belt 11 to the drum 3 side and impressing a transfer bias, a driven roller 15 energizing the belt 11 so as to be biased in the prescribed direction orthogonally crossing to the traveling direction and a regulation guide belt for preventing the bias 60 arranged by being opposed to the energizing side of the belt 11 and brought into slidable contact with the end part of the belt 11 to regulated the meandering of the belt 11. Then, when the energizing force of the transfer belt is defined as T, the thickness thereof is defined as (t), the contact length thereof with respect to the regulation guide for preventing the bias is defined as L and the elasticity of the transfer belt is defined as K, the condition of T/t.L.K≤0.0006 is satisfied.