A relative angle sensing device that senses a relative rotation angle between a first rotary shaft and a second rotary shaft, the relative angle sensing device comprising: a rotary component that is provided on the first rotary shaft and rotates together with the first rotary shaft; a supported component that generates a magnetic field and that is supported by the rotary component through an adhesive material; and a sensor that is provided in the second rotary shaft and outputs a value corresponding to the magnetic field generated by the supported component, characterized in that a convex part is provided in any one of the rotary component and the supported component and a concave part is provided in the other one of the rotary component and the supported component; and the supported component is supported by the rotary component through the adhesive material in a state where the convex part and the concave part are fitted with each other, and the rotative force of the first rotary shaft is transmitted by direct contact between the convex part and the concave part in the case of peeling of the adhesive material the length of the convex part or the concave part provided in the supported component in a shaft direction of the first rotary shaft is not less than length obtained by adding, to contact length between the convex part and the concave part sufficient to transmit the rotative force of the first rotary shaft to the supported component, length between an end part of the supported component on the second rotary shaft side and the second rotary shaft on which the supported component is placed as a result of falling down in the case of peeling of the adhesive material and/or rotates together with the second rotary shaft in a state where the supported component is supported by the rotary component through the adhesive material. a rotary component that is provided on the first rotary shaft and rotates together with the first rotary shaft; a supported component that generates a magnetic field and that is supported by the rotary component through an adhesive material; and a sensor that is provided in the second rotary shaft and outputs a value corresponding to the magnetic field generated by the supported component, a convex part is provided in any one of the rotary component and the supported component and a concave part is provided in the other one of the rotary component and the supported component; and the supported component is supported by the rotary component through the adhesive material in a state where the convex part and the concave part are fitted with each other, and the rotative force of the first rotary shaft is transmitted by direct contact between the convex part and the concave part in the case of peeling of the adhesive material the length of the convex part or the concave part provided in the supported component in a shaft direction of the first rotary shaft is not less than length obtained by adding, to contact length between the convex part and the concave part sufficient to transmit the rotative force of the first rotary shaft to the supported component, length between an end part of the supported component on the second rotary shaft side and the second rotary shaft on which the supported component is placed as a result of falling down in the case of peeling of the adhesive material and/or rotates together with the second rotary shaft in a state where the supported component is supported by the rotary component through the adhesive material.