A process for producing a polyether polyol which can form low viscosity rigid foamed synthetic resins excellent in strength, dimensional stability and flame retardancy, can give a low viscosity polyol system solution containing water instead of HFCs as a blowing agent with good miscibility with isocyanate compounds and can form good rigid foamed synthetic resins by spraying, which polyether polyol is produced by reacting a phenol component (molar ratio 1) selected from phenol and phenol derivatives having a hydrogen atom at one or more ortho-positions to the phenolic hydroxyl group, an aldehyde component (molar ratio 0.3 to 0.9) selected from formaldehyde and acetoaldehyde and an alkanolamine component (molar ratio 1.5 to 3.5) selected from monoethanolamine, diethanolamine and 1-amino-2-propanol and then adding an alkylene oxide to the resulting reaction product.