A fuser device comprising: a heating member; a pressure-applying member configured to form a nip portion in which a recording medium bearing an unfused toner image is clamped and the unfused toner image on the recording medium is melted and fused by causing the pressure-applying member to press against the heating member, the nip being formed by the heating member and the pressure-applying member; a coil for generating a magnetic flux for inductively heating the heating member, the coil being looped around the heating member along the lengthwise direction thereof; and a magnetic core, disposed near the coil, configured to guide the magnetic flux to an inductive heat-generating layer of the heating member, the magnetic core having: a first core section surrounding the coil and disposed along the widthwise direction of the recording medium orthogonally to the conveyance direction of the recording medium; and second core section members disposed at both ends in the widthwise direction within a hollow area which the loop of the coil forms, the second core section members being formed so that the cross-sectional area thereof in the conveyance direction grows progressively larger from the center of the widthwise direction towards the ends, characterized in that an inner surface is a surface of said second core section members facing towards the center with respect to the widthwise direction and is formed at one end of the second core section with respect to the widthwise direction and is a rectangle with long sides thereof extending in either a vertical or a horizontal direction. a heating member; a pressure-applying member configured to form a nip portion in which a recording medium bearing an unfused toner image is clamped and the unfused toner image on the recording medium is melted and fused by causing the pressure-applying member to press against the heating member, the nip being formed by the heating member and the pressure-applying member; a coil for generating a magnetic flux for inductively heating the heating member, the coil being looped around the heating member along the lengthwise direction thereof; and a magnetic core, disposed near the coil, configured to guide the magnetic flux to an inductive heat-generating layer of the heating member, the magnetic core having: a first core section surrounding the coil and disposed along the widthwise direction of the recording medium orthogonally to the conveyance direction of the recording medium; and second core section members disposed at both ends in the widthwise direction within a hollow area which the loop of the coil forms, the second core section members being formed so that the cross-sectional area thereof in the conveyance direction grows progressively larger from the center of the widthwise direction towards the ends, a first core section surrounding the coil and disposed along the widthwise direction of the recording medium orthogonally to the conveyance direction of the recording medium; and second core section members disposed at both ends in the widthwise direction within a hollow area which the loop of the coil forms, the second core section members being formed so that the cross-sectional area thereof in the conveyance direction grows progressively larger from the center of the widthwise direction towards the ends, characterized in that an inner surface is a surface of said second core section members facing towards the center with respect to the widthwise direction and is formed at one end of the second core section with respect to the widthwise direction and is a rectangle with long sides thereof extending in either a vertical or a horizontal direction.