workaholic; work fiend; devil for work
Describes someone who is extremely dedicated to work, often to an obsessive degree. The word 鬼 (oni, demon) is used metaphorically to emphasize intensity and single-mindedness.
He's a real workaholic; he works even on most of his days off.
That section chief is called a work fiend.
Compound of 仕事 (work) + の (possessive particle) + 鬼 (oni, demon). The metaphorical use of 鬼 to mean 'someone extremely devoted to something' is common in Japanese.