A rack bar supporting device for supporting a rack bar of a steering apparatus of a vehicle toward a pinion shaft, comprising; a rack bearing which is disposed at an opposite side of the pinion shaft in a state of contacting the rack bar; and a biasing assembly which provides a force of pushing the rack bearing along a biasing axis toward the rack bar so that the rack bearing pushes the rack bar to be engaged with the pinion shaft, wherein the biasing assembly comprises: an adjustment plug which can be fixed at a plurality of positions along the biasing axis; an adjustment assembly comprising an adjustment member which is formed to be able to move along the biasing axis relatively to the adjustment plug toward the rack bearing and a first biasing member which is connected respectively to the adjustment plug and the adjustment member so as to provide a force of pushing the adjustment member toward the rack bearing with respect to the adjustment plug; and a support plate assembly comprising a first support plate (251, 251a) which is supported by the rack bearing, a second support plate which is disposed to face the first support plate (251, 251a) in a state of being supported by the adjustment member, a second biasing member which is interposed between the first and the second support plates (251, 251a, 252) so as to provide a force of pushing the first support plate (251, 251a) along the biasing axis against the second support plate, and a connection structure which connects the first and the second support plates (251, 251a, 252) so as to restrict movement of the second support plate in a direction far away from the rack bearing, characterized in that the second support plate is installed at an initial installation state in a state of contacting none of the first support plate (251, 251a) and the connection structure in the biasing axis, so that gaps (d1, d2) exist respectively between the second support plate and the first support plate (251, 251a) and between the second support plate and a supporting part of the connection structure. an adjustment plug which can be fixed at a plurality of positions along the biasing axis; an adjustment assembly comprising an adjustment member which is formed to be able to move along the biasing axis relatively to the adjustment plug toward the rack bearing and a first biasing member which is connected respectively to the adjustment plug and the adjustment member so as to provide a force of pushing the adjustment member toward the rack bearing with respect to the adjustment plug; and a support plate assembly comprising a first support plate (251, 251a) which is supported by the rack bearing, a second support plate which is disposed to face the first support plate (251, 251a) in a state of being supported by the adjustment member, a second biasing member which is interposed between the first and the second support plates (251, 251a, 252) so as to provide a force of pushing the first support plate (251, 251a) along the biasing axis against the second support plate, and a connection structure which connects the first and the second support plates (251, 251a, 252) so as to restrict movement of the second support plate in a direction far away from the rack bearing, characterized in that the second support plate is installed at an initial installation state in a state of contacting none of the first support plate (251, 251a) and the connection structure in the biasing axis, so that gaps (d1, d2) exist respectively between the second support plate and the first support plate (251, 251a) and between the second support plate and a supporting part of the connection structure.