ATM(automated-teller machine) is a machine that receives and pays money directly. The double bill detector(DBD) module of an ATM detects double bills from its thickness. In this paper, the dynamic behavior of the DBD was analyzed, which cannot be simulated correctly by a package program of SAMCEF. And the displacements of the lever was measured for various bills experimentally. The measured dynamic behaviors were analyzed kinematically using a vector equation and simulated dynamically by the derivation of equation of motion with consideration of damping and stiffness of the DBD and bills. The analysis showed that the spring constant of the system is not so influential, instead, the contact angle between the lever and bill needs to be larger than the existing DBD in order to be laster and reliable DBD.