changes in a long driving and a problem with safety may be presented in comparative example k 16 in which the setting of the nco content was high the quantity of a polyol having an oxyethylene glycol unit was small and as a result the crosslinking effect due to the agglomeration was low and the hysteresis loss was high in the case that the mdi content is lowered as in the case of comparative example k 17 the chemical crosslinking density in the flexible foam resin is excessively high and the mechanical strength of the foam is deteriorated although a low hysteresis loss rate and a high durability can be achieved in the case that the number of functional groups of a polyol as a modifier is set low as in the case of comparative example k 18 the chemical crosslinking density in the flexible foam resin is lowered the hysteresis loss rate increases and the durability is deteriorated the modifier used for comparative example k 21 contains no polyoxyethylene unit and has a freezing point being out of the range of 10 to 45 u00b0 c and is liquid at normal temperature as a result the crosslinking effect due to the agglomeration was low and the hysteresis loss was high in comparative example.