A thermal fluid flow sensor comprising: - a semiconductor substrate; - a heating resistor and a measuring element that are formed in a diaphragm provided in the semiconductor substrate; - a first insulating film that is formed on each of the heating resistor and the measuring element; wherein the first insulating film is a silicon oxide film; - a first recess portion that is formed in an upper face of the first insulating film immediately above each of the heating resistor and the measuring element; and - a second insulating film that covers a side wall and a bottom face of the first recess portion, wherein the second insulating film is a silicon nitride film, characterized in that: the first recess portion is formed immediately below a second recess portion, and an opening width of the second recess portion is larger than an opening width of the first recess portion, wherein a side wide of the first recess portion does not overlap a side wall of second recess portion when seen in a plan view, that is, the first recess portion is formed at an inner side than the side wall of the second recess portion, and the side wall of the first recess portion and the side wall of the second recess portion are isolated from each other when seen in a plan view, wherein one step is formed using the bottom face of the second recess portion and the side wall of the first recess portion, wherein a third recess portion is formed on the upper face of the second insulating film along a surface shape of the first recess portion and the second recess portion, and star-like steps are formed on the side wall of the third recess portion, wherein the coatability of the second insulating film is improved by forming the film thickness of the second insulating film to be larger than any depth of the first and second recess portions. - a semiconductor substrate; - a heating resistor and a measuring element that are formed in a diaphragm provided in the semiconductor substrate; - a first insulating film that is formed on each of the heating resistor and the measuring element; wherein the first insulating film is a silicon oxide film; - a first recess portion that is formed in an upper face of the first insulating film immediately above each of the heating resistor and the measuring element; and - a second insulating film that covers a side wall and a bottom face of the first recess portion, wherein the second insulating film is a silicon nitride film, the first recess portion is formed immediately below a second recess portion, and an opening width of the second recess portion is larger than an opening width of the first recess portion, wherein a side wide of the first recess portion does not overlap a side wall of second recess portion when seen in a plan view, that is, the first recess portion is formed at an inner side than the side wall of the second recess portion, and the side wall of the first recess portion and the side wall of the second recess portion are isolated from each other when seen in a plan view, wherein one step is formed using the bottom face of the second recess portion and the side wall of the first recess portion, wherein a third recess portion is formed on the upper face of the second insulating film along a surface shape of the first recess portion and the second recess portion, and star-like steps are formed on the side wall of the third recess portion, wherein the coatability of the second insulating film is improved by forming the film thickness of the second insulating film to be larger than any depth of the first and second recess portions.