An indoor unit of an air conditioner is provided, comprising an air duct, an air outlet in communication with the air duct, and an air deflector for guiding the direction of air outflow of the air conditioner, with a volute tongue being provided at an end of the air duct proximately to the air outlet, wherein the indoor unit of the air conditioner further comprises an air deflector opening and closing mechanism in drive connection with the air deflector which is rotatably provided at the outer side of the air outlet. The indoor unit of the air conditioner can sealingly close the air outlet with respect to the air deflector and avoid any gap from being formed between the air deflector and the air outlet when the air deflector is closed.