An indole derivative represented by the following general formula (1): ##STR00001## wherein at least one of R.sup.1, R.sup.2, R.sup.3, and R.sup.4 represents an alkoxy group containing 1 to 20 carbon atoms, and other groups of the R.sup.1, R.sup.2, R.sup.3, and R.sup.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.sub.2).sub.nOH 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; and a drug and an agent for promoting differentiation of a stem cell containing such indole derivative or its salt as an effective component, wherein the indole derivative (1) has action of inducing differentiation of neural stem cell specifically into a neuron, and this indole derivative is useful as a prophylactic or therapeutic drug for brain dysfunction or neuropathy caused by loss or degeneration of the neuron.