Bonnet seal structure of swiveling work machine

A bonnet sealing structure for a swiveling work machine includes: a bonnet (36) configured to cover an engine (14) provided on a swivel stage (2) including: a fixed cover (22), and an open-close cover (24) which is swingably supported about a vertical shaft (77) and is openable and closable relative to the fixed cover (22); and a sealing member (84) through which an upper end portion of the open-close cover (24) is brought into contact with an upper end portion of the fixed cover (22) when the open-close cover (24) is closed. The bonnet sealing structure further includes a cover piece (81) protruding from one of an upper wall (65a) of the open-close cover (24) and an upper wall (33) of the fixed cover (22), and when the open-close cover (24) is closed, the cover piece (81) is located above the sealing member (84) and reaches a point above the other of the upper wall (33) of the fixed cover (22) and the upper wall (65a) of the open-close cover (21).