A turbocharger comprising: a turbine housing for housing a turbine wheel rotary driven by an exhaust gas; a bearing housing for housing a rotational shaft on which the turbine wheel is mounted, the bearing housing being adjacent to the turbine housing; and a wastegate portion including a bypass passage and a valve body, the bypass passage allowing the exhaust gas to bypass the turbine wheel, the valve body being able to adjust a flow rate of the exhaust gas flowing through the bypass passage, wherein, in the turbine housing, a scroll passage for supplying the exhaust gas to the turbine wheel is formed by casting, at least on a radially outer side of the turbine wheel, and wherein a fastening portion for fastening the turbine housing and the bearing housing is disposed in a circumferential entire region of the bearing housing, on a radially outer side of the scroll passage, wherein a scroll passage forming portion for forming the scroll passage is configured such that a side thereof facing the bearing housing is opened, wherein the turbocharger includes a back plate disposed to close at least a bearing-housing side open surface of the scroll passage forming portion, wherein the back plate extends radially outward up to the fastening portion, the back plate having a radially outer end part fixed to the fastening portion, wherein the fastening portion is composed of: a first fastening portion disposed in the turbine housing; a second fastening portion disposed in the bearing housing; and a fastening unit for fastening the first fastening portion and the second fastening portion, wherein the radially outer end part of the back plate is interposed by the first fastening portion and the second fastening portion, wherein the turbocharger further comprises an insulator of a donut shape with which a portion between the back plate and the second fastening portion is filled, and wherein a space (s) of a donut shape is formed between the insulator and the second fastening portion. a turbine housing for housing a turbine wheel rotary driven by an exhaust gas; a bearing housing for housing a rotational shaft on which the turbine wheel is mounted, the bearing housing being adjacent to the turbine housing; and a wastegate portion including a bypass passage and a valve body, the bypass passage allowing the exhaust gas to bypass the turbine wheel, the valve body being able to adjust a flow rate of the exhaust gas flowing through the bypass passage, wherein, in the turbine housing, a scroll passage for supplying the exhaust gas to the turbine wheel is formed by casting, at least on a radially outer side of the turbine wheel, and wherein a fastening portion for fastening the turbine housing and the bearing housing is disposed in a circumferential entire region of the bearing housing, on a radially outer side of the scroll passage, wherein a scroll passage forming portion for forming the scroll passage is configured such that a side thereof facing the bearing housing is opened, wherein the turbocharger includes a back plate disposed to close at least a bearing-housing side open surface of the scroll passage forming portion, wherein the back plate extends radially outward up to the fastening portion, the back plate having a radially outer end part fixed to the fastening portion, wherein the fastening portion is composed of: a first fastening portion disposed in the turbine housing; a second fastening portion disposed in the bearing housing; and a fastening unit for fastening the first fastening portion and the second fastening portion, wherein the radially outer end part of the back plate is interposed by the first fastening portion and the second fastening portion, wherein the turbocharger further comprises an insulator of a donut shape with which a portion between the back plate and the second fastening portion is filled, and wherein a space (s) of a donut shape is formed between the insulator and the second fastening portion. a first fastening portion disposed in the turbine housing; a second fastening portion disposed in the bearing housing; and a fastening unit for fastening the first fastening portion and the second fastening portion, wherein the radially outer end part of the back plate is interposed by the first fastening portion and the second fastening portion, wherein the turbocharger further comprises an insulator of a donut shape with which a portion between the back plate and the second fastening portion is filled, and wherein a space (s) of a donut shape is formed between the insulator and the second fastening portion.