PURPOSE: To obtain the titled compound useful as a photo-stabilizer of plastics, under mild reaction condition, with simple operation, in high yield, without causing the problem of waste gas, by reacting hydroxybenzoic acid with a phenol derivative using boron trifluoride as a catalyst.

CONSTITUTION: The objective compound of formula III is prepared by reacting the hydroxybenzoic acid compound of formula I (R1 and R2 are H, halogen, hydroxyl or alkoxy of CnH2n-1O- and at least one of R1 and R2 is hydroxyl; n is 1W18) with the phenol derivative of formula II (R3, R4, R5, R6 and R7 are H, halogen, hydroxyl or alkoxy of CnH2n-1O-, and at least one of R3, R4, R5, R6 and R7 is hydroxyl; n is 1W18) in the presence of boron trifluoride. The amount of the compound of formula I is preferably 0.8W1.2mol, especially 1.0W1.05mol per 1mol of the compound of formula II.