Provided is a method for producing (meth) acrylonitrile by using a fluidized bed reactor, where the (meth) acrylonitrilecan be stably produced over a long period of time and can maintain a reaction result. The method uses a fluidized bed reaction device having a fluidized bed reactor and a heat exchanger connected by a reaction gas outlet pipe of the fluidized bed reactor, and comprises a step ofintroducing a raw material gas into the fluidized bed reactor and performing an ammoxidation reaction in the presence of a catalyst to obtain a reaction gas; and a step of introducing the powder intothe reaction gas outlet pipe and discharging the reaction gas to the heat exchanger while cleaning the heat exchanger, wherein the reaction gas outlet pipe has a first portion which extends from an upper portion of the fluidized bed reactor in a height direction of the fluidized bed reactor to the highest point of the reaction gas outlet pipe, and a second portion connected to the highest point and extending to the heat exchanger, and the introduction position of the powder is a position in the second portion and lower than the highest point.