This paper introduces a new graph problem to find an Optimal Euler Circuit (OEC) in an Euler graph. OEC is defined as the Euler circuit that maximizes the sum of contiguous costs along it, where the contiguous cost is assigned for each of the two contiguous edges incident to a vertex. We prove that the OEC problem is NP-complete. A polynomial time algorithm will be presented for the case of a graph without vertex of degree greater than 4, and for the general case, a 1/4-approximation polynomial time algorithm will be proposed.