A moisture separator heater comprising: a casing having a tubular shape, extending in a horizontal direction and having both end portions closed; a manifold chamber provided in the casing and configured so that steam flows in an axial direction of the casing; a separator provided in the casing and in a longitudinal direction of the casing along the manifold chamber and configured to remove moisture in the steam; characterized in that the casing includes a body part having a cylindrical form and extending in the horizontal direction, the manifold chamber includes a partition plate provided in the body part, the manifold chamber is disposed on one side of the casing in a width direction of the body part, the partition plate, having a rectangular shape in a lateral view, extends from one end to the other end of the body part in the longitudinal direction and extends in a vertical direction such that both end portions in the up-and-down direction are fixed on an inner surface of the body part, the partition plate is provided with a plurality of slits at prescribed intervals in the axial direction of the casing, the separator extends in the axial direction of the casing and stands in the vertical direction, a heating tube bundle is provided in the casing and arranged in plates on a downstream side of the steam flowing out of the separator, the heating tube bundle extending in the axial direction of the casing and standing in the vertical direction, the heating tube bundle has a rectangular shape in a lateral view and has a longitudinal edge extending along the axial direction of the casing and a transverse edge extending in the vertical direction, the separator is formed in a plate-like shape having a rectangular shape in a lateral view, the separator is arranged so that a longitudinal edge of the plate-like shape extends along the axial direction and a transverse edge of the plate-like shape extends along the vertical direction, and the heating tube bundle is arranged along the plate like shape of the separator to extend in the axial direction and in the vertical direction. a casing having a tubular shape, extending in a horizontal direction and having both end portions closed; a manifold chamber provided in the casing and configured so that steam flows in an axial direction of the casing; a separator provided in the casing and in a longitudinal direction of the casing along the manifold chamber and configured to remove moisture in the steam; characterized in that the casing includes a body part having a cylindrical form and extending in the horizontal direction, the manifold chamber includes a partition plate provided in the body part, the manifold chamber is disposed on one side of the casing in a width direction of the body part, the partition plate, having a rectangular shape in a lateral view, extends from one end to the other end of the body part in the longitudinal direction and extends in a vertical direction such that both end portions in the up-and-down direction are fixed on an inner surface of the body part, the partition plate is provided with a plurality of slits at prescribed intervals in the axial direction of the casing, the separator extends in the axial direction of the casing and stands in the vertical direction, a heating tube bundle is provided in the casing and arranged in plates on a downstream side of the steam flowing out of the separator, the heating tube bundle extending in the axial direction of the casing and standing in the vertical direction, the heating tube bundle has a rectangular shape in a lateral view and has a longitudinal edge extending along the axial direction of the casing and a transverse edge extending in the vertical direction, the separator is formed in a plate-like shape having a rectangular shape in a lateral view, the separator is arranged so that a longitudinal edge of the plate-like shape extends along the axial direction and a transverse edge of the plate-like shape extends along the vertical direction, and the heating tube bundle is arranged along the plate like shape of the separator to extend in the axial direction and in the vertical direction.