A development device 4a is separated into a toner agitating portion 21 and a toner supply portion 22 by a boundary wall 23 in which a first opening 28 and a second opening 29 are formed. Inside the toner agitating portion 21, an agitation paddle 24 is rotatably supported, and inside the toner supply portion 22, components such as a development roller 25, a toner supply roller 16, and a regulation member 27 made of metal for regulating the thickness of a thin toner layer formed on the development roller 25 and for electrically charging toner are provided. A groove 27a is formed in the regulation member 27 all along the length thereof.