A heat flux sensor is provided with a main body which detects heat flux, and filling members. The main body has a first surface. The first surface has an uneven shape, with a plurality of concave portions and a plurality of convex portions. The filling members are filled in the plurality of concave portions. Surfaces of the filling members constitutes a part of an outer surface of the heat flux sensor. The degree of flatness of the outer surface is higher than the degree of flatness of the first surface of the main body.