A process for producing a seat using a flexible polyurethane foam, comprising a process for producing a polyether polyol comprising: a step of subjecting an alkylene oxide that is only propylene oxide to ring-opening addition polymerization to an initiator having an average number of hydroxy groups of from 2 to 8, in the presence of a double metal cyanide complex catalyst, to obtain an intermediate polyol; and a step of subjecting ethylene oxide to ring-opening addition polymerization in an amount of from 1 to 23 mol per 1 mol of the initiator to the intermediate polyol, in the presence of an alkali metal hydroxide as a polymerization catalyst, to obtain a polyoxyalkylene polyol having a hydroxy value of from 5 to 25 mgKOH/g; wherein the intermediate polyol is a polyol obtained by the following initial activation step (a) and the following additional polymerization step (b): initial activation step (a): a step of supplying and reacting to a reaction fluid containing the above initiator and the above double metal cyanide complex catalyst, a part of the above alkylene oxide so as to be in an amount of from 5 to 20 parts by mass per 100 parts by mass of the initiator contained in the above reaction fluid, wherein the initial temperature of the reaction fluid immediately before supplying the above alkylene oxide is from 120 to 165°C, and the maximum temperature of the above reaction fluid in the initial activation step (a) is higher by from 15 to 50°C than the above initial temperature, and additional polymerization step (b): a step of additionally supplying the alkylene oxide after the above initial activation step (a), and carrying out ring-opening addition polymerization in the presence of the double metal cyanide complex catalyst. a step of subjecting an alkylene oxide that is only propylene oxide to ring-opening addition polymerization to an initiator having an average number of hydroxy groups of from 2 to 8, in the presence of a double metal cyanide complex catalyst, to obtain an intermediate polyol; and a step of subjecting ethylene oxide to ring-opening addition polymerization in an amount of from 1 to 23 mol per 1 mol of the initiator to the intermediate polyol, in the presence of an alkali metal hydroxide as a polymerization catalyst, to obtain a polyoxyalkylene polyol having a hydroxy value of from 5 to 25 mgKOH/g; initial activation step (a): a step of supplying and reacting to a reaction fluid containing the above initiator and the above double metal cyanide complex catalyst, a part of the above alkylene oxide so as to be in an amount of from 5 to 20 parts by mass per 100 parts by mass of the initiator contained in the above reaction fluid, wherein the initial temperature of the reaction fluid immediately before supplying the above alkylene oxide is from 120 to 165°C, and the maximum temperature of the above reaction fluid in the initial activation step (a) is higher by from 15 to 50°C than the above initial temperature, and additional polymerization step (b): a step of additionally supplying the alkylene oxide after the above initial activation step (a), and carrying out ring-opening addition polymerization in the presence of the double metal cyanide complex catalyst.