A method for producing γ-Glu-Val and/or a salt thereof, the method comprising the following step: a step of allowing a mutant glutamate-cysteine ligase to act on Glu and Val to generate γ-Glu-Val, wherein the mutant glutamate-cysteine ligase has a mutation in the amino acid sequence of a wild-type glutamate-cysteine ligase, and has the γ-glutamylvaline synthetase activity, wherein the mutant glutamate-cysteine ligase shows a ratio of γ-glutamylvaline synthetase activity to γ-glutamylglycine synthetase activity of 0.7 or higher, and wherein the mutation includes a mutation corresponding to one or more mutations selected from those mentioned below: L135(I, F, M, V, G, A, W, K, H, R, C, N, S, T), Q144(F, A, N, S, D, T, R, H, G, K, Y, W, C, M, P, V, L, I), Y241, N243(I, W, K, R, H), Y300(A, H, R, K), wherein the wild-type glutamate-cysteine ligase is a protein defined in (a), (b), or (c) mentioned below: (a) a protein comprising the amino acid sequence of SEQ ID NO: 2; (b) a protein comprising the amino acid sequence of SEQ ID NO: 2 but including substitution, deletion, insertion, or addition of 1 to 10 amino acid residues; (c) a protein comprising an amino acid sequence showing an identity of 90% or higher to the amino acid sequence of SEQ ID NO: 2. a step of allowing a mutant glutamate-cysteine ligase to act on Glu and Val to generate γ-Glu-Val, wherein the mutant glutamate-cysteine ligase has a mutation in the amino acid sequence of a wild-type glutamate-cysteine ligase, and has the γ-glutamylvaline synthetase activity, wherein the mutant glutamate-cysteine ligase shows a ratio of γ-glutamylvaline synthetase activity to γ-glutamylglycine synthetase activity of 0.7 or higher, and wherein the mutation includes a mutation corresponding to one or more mutations selected from those mentioned below: L135(I, F, M, V, G, A, W, K, H, R, C, N, S, T), Q144(F, A, N, S, D, T, R, H, G, K, Y, W, C, M, P, V, L, I), Y241, N243(I, W, K, R, H), Y300(A, H, R, K), wherein the wild-type glutamate-cysteine ligase is a protein defined in (a), (b), or (c) mentioned below: (a) a protein comprising the amino acid sequence of SEQ ID NO: 2; (b) a protein comprising the amino acid sequence of SEQ ID NO: 2 but including substitution, deletion, insertion, or addition of 1 to 10 amino acid residues; (c) a protein comprising an amino acid sequence showing an identity of 90% or higher to the amino acid sequence of SEQ ID NO: 2. wherein the mutant glutamate-cysteine ligase has a mutation in the amino acid sequence of a wild-type glutamate-cysteine ligase, and has the γ-glutamylvaline synthetase activity, wherein the mutant glutamate-cysteine ligase shows a ratio of γ-glutamylvaline synthetase activity to γ-glutamylglycine synthetase activity of 0.7 or higher, and wherein the mutation includes a mutation corresponding to one or more mutations selected from those mentioned below: L135(I, F, M, V, G, A, W, K, H, R, C, N, S, T), Q144(F, A, N, S, D, T, R, H, G, K, Y, W, C, M, P, V, L, I), Y241, N243(I, W, K, R, H), Y300(A, H, R, K), wherein the wild-type glutamate-cysteine ligase is a protein defined in (a), (b), or (c) mentioned below: (a) a protein comprising the amino acid sequence of SEQ ID NO: 2; (b) a protein comprising the amino acid sequence of SEQ ID NO: 2 but including substitution, deletion, insertion, or addition of 1 to 10 amino acid residues; (c) a protein comprising an amino acid sequence showing an identity of 90% or higher to the amino acid sequence of SEQ ID NO: 2. L135(I, F, M, V, G, A, W, K, H, R, C, N, S, T), Q144(F, A, N, S, D, T, R, H, G, K, Y, W, C, M, P, V, L, I), Y241, N243(I, W, K, R, H), Y300(A, H, R, K), wherein the wild-type glutamate-cysteine ligase is a protein defined in (a), (b), or (c) mentioned below: (a) a protein comprising the amino acid sequence of SEQ ID NO: 2; (b) a protein comprising the amino acid sequence of SEQ ID NO: 2 but including substitution, deletion, insertion, or addition of 1 to 10 amino acid residues; (c) a protein comprising an amino acid sequence showing an identity of 90% or higher to the amino acid sequence of SEQ ID NO: 2. (a) a protein comprising the amino acid sequence of SEQ ID NO: 2; (b) a protein comprising the amino acid sequence of SEQ ID NO: 2 but including substitution, deletion, insertion, or addition of 1 to 10 amino acid residues; (c) a protein comprising an amino acid sequence showing an identity of 90% or higher to the amino acid sequence of SEQ ID NO: 2.