PURPOSE: To inhibit attaching of soil to a cover by closing a hole for piercing props of a wing hiller and inhibiting invasion of soil by providing an elastic cover on a cover for closing a hole of a rear plate cover.

CONSTITUTION: A gap T approximately corresponding to a protruding amount in a hooking part is formed between a rear plate cover 12 and an elastic cover 14 and the elastic cover 14 is fixed in an extended state, then flinging soil mud back in colliding to an inner face. The rear plate cover 12 and the elastic cover 14 are hooked at an engaging part and a hooking part around piercing holes 18 and 39 to props of a wing hiller to inhibit invasion of soil into between the both. In the case of not using the wing hiller, a hole 18 in the rear plate cover 12 is closed with a cover 19 and a hole 39 in the elastic cover 14 is closed with an elastic cover 42 fitted on the cover 19, then invasion of soil into between the both covers 19 and 42 is inhibited. Thereby, attaching of soil to the cover 19 is also inhibited.