A flow rate sensor comprises a fluid passage for a fluid to flow therealong; a main body structure arranged so as to turn its front end to the upstream side of the fluid and coaxially positioned within the fluid passage; and a detecting element disposed between the main body structure and the internal surface of the fluid passage. The main body structure is formed such that its cross section perpendicular to its central axis, at first becomes gradually larger from its front end towards its rear end and then becomes gradually smaller. The detecting element includes a ceramic substrate coated with a film made of a thermo-sensitive electrically resistant material. The film is formed into a meander pattern so as to form a flow rate detecting resistance. The detecting element further includes a fluid temperature compensating resistance.