A method of producing a solution composition containing a monoetherified product, comprising: a step of reacting a hydroquinone compound of formula (I), shown below, where R 1 , R 2 , R 3 , and R 4 each represent, independently of one another, a hydrogen atom, a halogen atom, an optionally substituted alkyl group having a carbon number of 1 to 6, a -C(=O)-O-R' group, or a -C(=O)-R' group, with R' representing an optionally substituted alkyl group having a carbon number of 1 to 6, and a hydroxyl group-containing etherifying agent of formula (II), shown below, HO-R 5 -X (II) where R 5 represents an optionally substituted alkylene group having a carbon number of 1 to 20 and X represents a leaving group, in a two-phase system including an alkaline aqueous solution and a hydrophobic ether solvent, wherein the hydrophobic ether solvent is an ether solvent for which solubility in 100 g of water at 25°C is 10 g/100 g-H 2 O or less, and in the absence of a phase transfer catalyst and a hydrophilic organic solvent, wherein in the absence of a phase transfer catalyst and a hydrophilic organic solvent means that the total amount of phase transfer catalyst and hydrophilic organic solvent relative to the total of the alkaline aqueous solution and the hydrophobic ether solvent is 0.01 mass% or less; and a step of, after the step , separating and removing an aqueous phase of a two-phase reaction liquid by liquid separation, and washing an organic phase using an aqueous solution containing a neutral inorganic salt and a hydroxide of an alkali metal or alkaline earth metal or a phosphate of an alkali metal or alkaline earth metal, wherein the monoetherified product is a monoetherified product of formula (III), shown below, where R 1 to R 5 represent the same as above. a step of reacting a hydroquinone compound of formula (I), shown below, where R 1 , R 2 , R 3 , and R 4 each represent, independently of one another, a hydrogen atom, a halogen atom, an optionally substituted alkyl group having a carbon number of 1 to 6, a -C(=O)-O-R' group, or a -C(=O)-R' group, with R' representing an optionally substituted alkyl group having a carbon number of 1 to 6, and a hydroxyl group-containing etherifying agent of formula (II), shown below, HO-R 5 -X (II) where R 5 represents an optionally substituted alkylene group having a carbon number of 1 to 20 and X represents a leaving group, in a two-phase system including an alkaline aqueous solution and a hydrophobic ether solvent, wherein the hydrophobic ether solvent is an ether solvent for which solubility in 100 g of water at 25°C is 10 g/100 g-H 2 O or less, and in the absence of a phase transfer catalyst and a hydrophilic organic solvent, wherein in the absence of a phase transfer catalyst and a hydrophilic organic solvent means that the total amount of phase transfer catalyst and hydrophilic organic solvent relative to the total of the alkaline aqueous solution and the hydrophobic ether solvent is 0.01 mass% or less; and a step of, after the step , separating and removing an aqueous phase of a two-phase reaction liquid by liquid separation, and washing an organic phase using an aqueous solution containing a neutral inorganic salt and a hydroxide of an alkali metal or alkaline earth metal or a phosphate of an alkali metal or alkaline earth metal, wherein the monoetherified product is a monoetherified product of formula (III), shown below, where R 1 to R 5 represent the same as above. <EOS>