To reduce an opening and closing operation load in a venetian blind in which a lifting/lowering cord is disposed at a position offset from a central portion in a back and forth direction of a slat. At least one lifting/lowering cord is disposed along one vertical cord of front and back vertical cords and introduced into a head box from one side in a back and forth direction of the head box, and the other lifting/lowering cord is disposed along the other vertical cord and introduced into the head box from the other side in the back and forth direction of the head box. All the lifting/lowering cords introduced into the head box from the different sides in the back and forth direction of the head box are wound in the same direction by the lifting/lowering drums to which the respective ends of the lifting/lowering cords are connected.