A method for producing an objective substance, the method comprising the following step: producing the objective substance by using a microorganism having an ability to produce the objective substance, wherein the microorganism has been modified to have an O-methyltransferase gene derived from a bacterium belonging to the genus Niastella, wherein the O-methyltransferase gene is a gene encoding a protein selected from a group consisting of: (a) a protein comprising the amino acid sequence of SEQ ID NO: 141, (b) a protein comprising the amino acid sequence of SEQ ID NO: 141 but that includes substitution, deletion, insertion, and/or addition of 1 to 10 amino acid residues, and wherein said protein has O-methyltransferase activity, (c) a protein comprising an amino acid sequence having an identity of 90% or higher to the amino acid sequence of SEQ ID NO: 141, and wherein said protein has O-methyltransferase activity, and (d) a protein as defined in (a), (b), or (c) but having a specific mutation, wherein the specific mutation is a mutation at an amino acid residue selected from the group consisting of D21, L31, M36, S42, L67, Y90, P144, and combinations thereof, and wherein the objective substance is selected from the group consisting of vanillin, vanillic acid, ferulic acid, guaiacol, 4-vinylguaiacol, 4-ethylguaiacol, and combinations thereof. producing the objective substance by using a microorganism having an ability to produce the objective substance, wherein the microorganism has been modified to have an O-methyltransferase gene derived from a bacterium belonging to the genus Niastella, wherein the O-methyltransferase gene is a gene encoding a protein selected from a group consisting of: (a) a protein comprising the amino acid sequence of SEQ ID NO: 141, (b) a protein comprising the amino acid sequence of SEQ ID NO: 141 but that includes substitution, deletion, insertion, and/or addition of 1 to 10 amino acid residues, and wherein said protein has O-methyltransferase activity, (c) a protein comprising an amino acid sequence having an identity of 90% or higher to the amino acid sequence of SEQ ID NO: 141, and wherein said protein has O-methyltransferase activity, and (d) a protein as defined in (a), (b), or (c) but having a specific mutation, wherein the specific mutation is a mutation at an amino acid residue selected from the group consisting of D21, L31, M36, S42, L67, Y90, P144, and combinations thereof, and (a) a protein comprising the amino acid sequence of SEQ ID NO: 141, (b) a protein comprising the amino acid sequence of SEQ ID NO: 141 but that includes substitution, deletion, insertion, and/or addition of 1 to 10 amino acid residues, and wherein said protein has O-methyltransferase activity, (c) a protein comprising an amino acid sequence having an identity of 90% or higher to the amino acid sequence of SEQ ID NO: 141, and wherein said protein has O-methyltransferase activity, and (d) a protein as defined in (a), (b), or (c) but having a specific mutation, wherein the specific mutation is a mutation at an amino acid residue selected from the group consisting of D21, L31, M36, S42, L67, Y90, P144, and combinations thereof, and wherein the objective substance is selected from the group consisting of vanillin, vanillic acid, ferulic acid, guaiacol, 4-vinylguaiacol, 4-ethylguaiacol, and combinations thereof.