A development device has a casing configured with a first frame and a second frame. The first frame includes attachment surfaces to which a regulating member is attached. A facing surface of the second frame is disposed between the attachment surfaces, in a predetermined plane including the attachment surfaces, and configured to face a portion of the regulating member between the attachment surfaces. A seal member is disposed between the regulating member and opposite surfaces including the attachment surfaces and the facing surface, and configured to seal first gaps each of which is formed between the facing surface and one of the attachment surfaces, in a first direction parallel to a rotational axis of a developing member rotatably supported by the casing.