The ether synthesizing reaction between 1-butyl bromide and phenol was investigated in a batch reactor and a continuous flow reactor. When polyethylene glycol (average molecular weight 3000) was used as a phase transfer catalyst and dodecane as an organic solvent, the third liquid phase was formed at high concentrations of potassium hydroxide in the aqueous phase. The volume of the third phase increased with the addition of phenol, but the base strength decreased. As a result, the production rate of ether in a batch reactor had a peak with respect to the concentration of phenol. Phase separable type continuous stirred tank reactor (CSTR) was devised as a continuous flow reactor to improve the efficiency of the third liquid phase. Reactants were supplied directly to the third phase and the region close to the third phase was stirred. The organic solvent, including the product butylphenyl ether, was continuously withdrawn. The reaction rate showed the same behavior in response to phenol concentration in the continuous flow reactor as in the batch reactor. The steady state operation could be performed when the supplied phenol was completely consumed.