[Object] The main object of the present invention is to provide a fibrosis inhibitor.[Solving Means] The present invention relates to a fibrosis inhibitor containing the heterocyclic derivative represented by the following general formula (1) or a pharmaceutically acceptable salt thereof as an active ingredient; In the formula (1), R1 and R2 are the same or different and each represents an optionally substituted aryl; R3 and R4 are the same or different and each represents hydrogen atom or alkyl; R5 represents hydrogen atom, alkyl or halogen atom; Y represents N or N→O; A represents NR6, and R6 represents hydrogen atom, alkyl, etc.; D represents alkylene or alkenylene which is optionally substituted with hydroxy; E represents phenylene or a single bond; G represents O, S, etc.; and Q represents carboxy, alkoxycarbonyl, etc.