A portable power working machine including a body portion, a cover attached to cover a predetermined region of the body portion, a guide bar held between the body portion and the cover, a bolt passed through an elongated hole or an elongated groove provided in the guide bar and through a bolt insertion hole provided in the cover, the bolt being securely embedded in the body portion, a fastening nut threadably engaged with the bolt so as to compressively hold the guide bar between the body portion and the cover, and a structure for preventing the falling of the fastening nut, the structure comprising: a nut locking member, the nut locking member being elastically deformable and adapted to be inserted through the bolt insertion hole from a back surface side of the cover so that movement of the nut locking member to a front surface side is prevented; and a nut anchor member provided on the nut on a seating surface side thereof, the nut anchor member being adapted to be inserted through the bolt insertion hole and being relatively difficult to elastically deform with respect to the nut locking member, wherein: the nut anchor member includes a hook portion formed continuous therewith, and the nut locking member includes at least one locking claw portion formed continuous therewith, the locking claw portion being adapted to allow the hook portion to be locked by the nut locking member, and wherein the nut locking member has a split cylindrical shape with a plurality of leg-like piece portions, each leg-like piece portion has formed at an end thereof the locking claw portion, and the nut locking member is made of a sheet metal. a nut locking member, the nut locking member being elastically deformable and adapted to be inserted through the bolt insertion hole from a back surface side of the cover so that movement of the nut locking member to a front surface side is prevented; and a nut anchor member provided on the nut on a seating surface side thereof, the nut anchor member being adapted to be inserted through the bolt insertion hole and being relatively difficult to elastically deform with respect to the nut locking member, wherein: the nut anchor member includes a hook portion formed continuous therewith, and the nut locking member includes at least one locking claw portion formed continuous therewith, the locking claw portion being adapted to allow the hook portion to be locked by the nut locking member, and wherein the nut locking member has a split cylindrical shape with a plurality of leg-like piece portions, each leg-like piece portion has formed at an end thereof the locking claw portion, and the nut locking member is made of a sheet metal. the nut anchor member includes a hook portion formed continuous therewith, and the nut locking member includes at least one locking claw portion formed continuous therewith, the locking claw portion being adapted to allow the hook portion to be locked by the nut locking member, and wherein the nut locking member has a split cylindrical shape with a plurality of leg-like piece portions, each leg-like piece portion has formed at an end thereof the locking claw portion, and the nut locking member is made of a sheet metal.