A heat radiation structure of an electric apparatus that comprises a motor device including: a motor section including a shaft whose one end projects from a housing for the electric device; a rotational position sensor disposed outside of the housing for detecting a rotational position of the shaft; a circuit substrate having a plurality of electronic components (33 to 39) mounted thereon and disposed more outward than the rotational position sensor with respect to the housing, a circuit substrate forming a signal processing circuit for signal processing based on an output signal output from the rotational position sensor; and a casing fixed to the housing and covering the rotational position sensor and the circuit substrate, characterized in that: the casing includes a casing body formed of an electrical insulating material and an electromagnetic wave shielding member made of a metal and disposed on an inner wall surface of the casing body without being in contact with the housing; the casing body (9,109) is formed with an opening portion (9c,109c) opened toward the housing (17,117), and includes an opposed wall portion (9a,109a) opposed to the opening portion (9c,109c); the electromagnetic wave shielding member is attached to the opposed wall portion (9a,109a); and a heat conductive member having electrical insulating and heat conductive properties and flexibility is disposed between the circuit substrate and the electromagnetic wave shielding member and closely attached to the electromagnetic wave shielding member. a motor section including a shaft whose one end projects from a housing for the electric device; a rotational position sensor disposed outside of the housing for detecting a rotational position of the shaft; a circuit substrate having a plurality of electronic components (33 to 39) mounted thereon and disposed more outward than the rotational position sensor with respect to the housing, a circuit substrate forming a signal processing circuit for signal processing based on an output signal output from the rotational position sensor; and a casing fixed to the housing and covering the rotational position sensor and the circuit substrate, characterized in that: the casing includes a casing body formed of an electrical insulating material and an electromagnetic wave shielding member made of a metal and disposed on an inner wall surface of the casing body without being in contact with the housing; the casing body (9,109) is formed with an opening portion (9c,109c) opened toward the housing (17,117), and includes an opposed wall portion (9a,109a) opposed to the opening portion (9c,109c); the electromagnetic wave shielding member is attached to the opposed wall portion (9a,109a); and a heat conductive member having electrical insulating and heat conductive properties and flexibility is disposed between the circuit substrate and the electromagnetic wave shielding member and closely attached to the electromagnetic wave shielding member. the casing includes a casing body formed of an electrical insulating material and an electromagnetic wave shielding member made of a metal and disposed on an inner wall surface of the casing body without being in contact with the housing; the casing body (9,109) is formed with an opening portion (9c,109c) opened toward the housing (17,117), and includes an opposed wall portion (9a,109a) opposed to the opening portion (9c,109c); the electromagnetic wave shielding member is attached to the opposed wall portion (9a,109a); and a heat conductive member having electrical insulating and heat conductive properties and flexibility is disposed between the circuit substrate and the electromagnetic wave shielding member and closely attached to the electromagnetic wave shielding member.