To reduce cost by eliminating a conventional bearing while securing dustproofing for a bearing box of a movable cover, since there is the problem of increasing cost due to conventionally using a bearing for securing the dustproofing and smooth opening-closing operation of this movable cover, though the lower side of a circular cutting blade, projecting to an under surface of a base placed on a cutting material, is covered with the movable cover, for example, in a portable cutting machine, that is, a cutting machine by which a user performs cutting work by moving by holding with hand.

This dustproof structure enhances the dustproofing of a movable cover support part by interposing a ring spring 31 between a support annular ring part 25b and an axial direction regulating part 19, while setting a necessary and sufficient clearance for smoothly operating the movable cover 25 for opening-closing, between the support annular ring part 25b of the movable cover 25 and a rotary boss part 17a of a bearing box 17.