A diluted solution of a synthetic fiber treatment agent, comprising: a synthetic fiber treatment agent including a smoothing agent, a nonionic surfactant, and an antistatic agent; and a volatile diluent, wherein where the total content of the synthetic fiber treatment agent and the volatile diluent in the diluted solution is taken as 100% by mass, the proportion of the synthetic fiber treatment agent is 60% by mass or more and less than 100% by mass, and the proportion of the volatile diluent is greater than 0% by mass and equal to or less than 40% by mass, the diluted solution has a kinematic viscosity at 30°C within the range of greater than 0 mm 2 /s to 70 mm 2 /s as determined by measurement on a Cannon-Fenske viscometer, wherein the smoothing agent is selected from at least one or more of the group consisting of ester compounds of aliphatic monohydric alcohols and aliphatic monocarboxylic acids; ester compounds of aliphatic polyhydric alcohols and aliphatic monocarboxylic acids; ester compounds of aliphatic monohydric alcohols and aliphatic polycarboxylic acids; ester compounds of aromatic monohydric alcohols and aliphatic monocarboxylic acids; ester compounds of aromatic polyhydric alcohols and aliphatic monocarboxylic acids; ester compounds of aliphatic monohydric alcohols and aromatic polycarboxylic acids; natural fats and oils; and mineral oils, wherein the nonionic surfactant is selected from at least one or more of the group consisting of compounds obtained by adding an alkylene oxide having 2 to 4 carbon atoms; polyhydric alcohol partial ester type nonionic surfactants; polyoxyalkylene polyhydric alcohol fatty acid ester type nonionic surfactants; fatty acid amide type nonionic surfactants; polyoxyalkylene fatty acid amide type nonionic surfactants; higher fatty acids; and higher alcohols, wherein the antistatic agent is selected from at least one or more of the group consisting of fatty acid salts; organic sulfonic acid salts; organic sulfuric acid salts; organic phosphoric acid salts; amphoteric compounds; and quaternary ammonium salts, wherein the volatile diluent is selected from at least one or more of the group consisting of water; organic solvents; and low viscosity mineral oils. a synthetic fiber treatment agent including a smoothing agent, a nonionic surfactant, and an antistatic agent; and a volatile diluent, wherein where the total content of the synthetic fiber treatment agent and the volatile diluent in the diluted solution is taken as 100% by mass, the proportion of the synthetic fiber treatment agent is 60% by mass or more and less than 100% by mass, and the proportion of the volatile diluent is greater than 0% by mass and equal to or less than 40% by mass, the diluted solution has a kinematic viscosity at 30°C within the range of greater than 0 mm 2 /s to 70 mm 2 /s as determined by measurement on a Cannon-Fenske viscometer, wherein the smoothing agent is selected from at least one or more of the group consisting of ester compounds of aliphatic monohydric alcohols and aliphatic monocarboxylic acids; ester compounds of aliphatic polyhydric alcohols and aliphatic monocarboxylic acids; ester compounds of aliphatic monohydric alcohols and aliphatic polycarboxylic acids; ester compounds of aromatic monohydric alcohols and aliphatic monocarboxylic acids; ester compounds of aromatic polyhydric alcohols and aliphatic monocarboxylic acids; ester compounds of aliphatic monohydric alcohols and aromatic polycarboxylic acids; natural fats and oils; and mineral oils, wherein the nonionic surfactant is selected from at least one or more of the group consisting of compounds obtained by adding an alkylene oxide having 2 to 4 carbon atoms; polyhydric alcohol partial ester type nonionic surfactants; polyoxyalkylene polyhydric alcohol fatty acid ester type nonionic surfactants; fatty acid amide type nonionic surfactants; polyoxyalkylene fatty acid amide type nonionic surfactants; higher fatty acids; and higher alcohols, wherein the antistatic agent is selected from at least one or more of the group consisting of fatty acid salts; organic sulfonic acid salts; organic sulfuric acid salts; organic phosphoric acid salts; amphoteric compounds; and quaternary ammonium salts, wherein the volatile diluent is selected from at least one or more of the group consisting of water; organic solvents; and low viscosity mineral oils.