1. A “Device for Reducing the Width of Graph” which reduces the width of the Binary Decision Diagram for Characteristic Function (BDD_for_CF), where BDD_for_CF is a characteristic function χ(X,Y) defined in Equation (1), X=(x 1 ,. .. ,x n ) (nεN, N is a set of natural numbers) denotes input variables, Y=(y 0 ,. .. , y m−1 ) (m≧2, mεN) denotes the output variables of a multiple-output logic function F(X), and F(X)=(ƒ 0 (X),. .. ,ƒ m−1 (X)) is an incompletely specified function to the output including don't care, comprising: (A) “Means to Store Node Table” storing the node table which is the table of node data that consists of the labels of variables and pairs of edges e 0 (v i ) and e 1 (v i ), where the labels of variables are labels given to the variables z i (z i ε(X∪Y)) corresponding to said each non-terminal node v i in the BDD_for_CF of the multiple-output logic function F(X), and a pair of edges e 0 (v i ) and e 1 (v i ) that points the next transition child node(s) when the input values of z i (z i ε(X∪Y)) are 0 and 1; (B) “Means to Find the Dividing Lines” setting the height of the partition lev which partitions BDD_for_CF represented by said node table stored in said “Means to Store Node Table”; (C) “Means to Generate Column Functions” generating a column function which represents the column of the decomposition chart derived by the functional decomposition from said node table stored in said “Means to Store Node Table”, where the decomposition is obtained by partitioning said BDD_for_CF by said height of the partition lev set by said “Means to Find the Dividing Lines”; and (D) “Means to Reconstruct Assigned BDD” which assigns the constants to the don't care in the compatible column functions of column function generated by said “Means to Generate Column Functions”, and consequently makes these compatible column functions to the identical column functions (hereafter, assigned column functions), and reconstructs said BDD_for_CF using the new assigned column function, and finally updates the node table in said “Means to Store Node Table”, [Equation 1] χ ⁡ ( X , Y ) = ⩓ m - 1 i = 0 ⁢ { y _ i ⁢ f i_ ⁢ 0 ⋁ y i ⁢ f i_ ⁢ 1 ⋁ f i_d } ( 1 ) where ƒ i — 0 , ƒ i — 1 ,ƒ i — d are the OFF function, the ON function and the DC function defined in Equation (2), respectively, [Equation 2] f i_ ⁢ 0 ⁡ ( X ) = { 1 ( X ∈ f i - 1 ⁡ ( 0 ) ) 0 ( otherwise ) , f i_ ⁢ 1 ⁡ ( X ) = { 1 ( X ∈ f i - 1 ⁡ ( 1 ) ) 0 ( otherwise ) , f i_d ⁡ ( X ) = { 1 ( X ∈ f i - 1 ⁡ ( d ) ) 0 ( otherwise ). ( 2 )