1. An indole derivative represented by the following general formula (1): wherein at least one of R 1 , R 2 , R 3 and R 4 represents an alkoxy group containing 1 to 20 carbon atoms, and other groups of the R 1 , R 2 , R 3 , and R 4 represent hydrogen, an alkyl group containing 1 to 6 carbon atoms, acetyl group, or hydroxyl group; and either one of X and Y represents —(CH 2 ) n OH wherein n is an integer of 0 to 30, and the other one of the X and Y represents hydrogen atom; or a salt thereof.