An insulating bobbin for disposition between a stator core and a stator winding of a stator of a rotating machine, the stator winding being wound around the stator core by way of the insulating bobbin, the insulating bobbin comprising a coil end section at which an end portion of the stator winding is disposed, a slot side section forming a slot for winding thereon the stator winding, and a corner section having a curved surface and connecting between the coil end portion and the slot side portion, the corner section having a first radius of curvature at a side closer to the coil end section and a second radius of curvature at a side closer to the slot side section, the second radius of curvature being larger than the first radius of curvature.