Disclosed is an electronic payment device and its method using an balanced binary tree for calculating a first data Xk of current consumption or a plurality of root values Rq from an initial value Xn of the amount of money that user purchases. The first data Xk is correspondent to a position value j with a respective relation of j=n−k+1. The first data Xk is generated according to the value of the binary code dm−1dm−2 . . . d1d0 of the position value j being a 0 or 1, by formula Xk=hd0(. . . (hdm−2(hdm−1(Xn)))). If the binary code di=0, i=m−1, m−2, . . . , 1, 0, then the first one-way function h0 is used as an operating function. Otherwise, the second one-way function h1 is operated when the binary code di=1. Moreover, a function operating device for an electronic payment using an balanced binary tree is also disclosed. The present invention requires less calculation than that in the prior art, and thus is more effective.