1. A vehicle seat comprising: a seat cushion on which a person is seated; and a seat back erected at a rear end of the seat cushion, the seat back being rotatably configured with respect to the seat cushion, wherein: a seat back frame forming a skeletal outline for the seat back includes an upper frame part formed of a pipe material, the upper frame part being placed on an upper part of the seat back frame, a lower frame part formed of a pipe material, the lower frame part being placed on a lower part of the seat back frame, a first side frame part formed of a plate member, the first side frame part being coupled between the upper frame part and the lower frame part on one side in a width direction of the vehicle seat, a second side frame part formed of a plate member, the second side frame part being coupled between the upper frame part and the lower frame part on the other side in the width direction, wherein the upper frame part, the lower frame part, the first side frame part, and the second side frame part are formed as separate components, and the upper frame part, the lower frame part, the first side frame part, and the second side frame part are joined to configure the seat back frame; the lower frame part is provided in a manner that a rotational axis about which the seat back is rotated with respect to the seat cushion is located on an inner side of the lower frame part; and a pipe bracket is provided between the lower frame part and the second side frame part, the pipe bracket joining the lower frame part to the second side frame part, characterized in that a lower panel is placed near the lower frame part; one end portion in the width direction of the lower panel is joined to the first side frame part, the other end portion in the width direction is joined to the second side frame part, and a lower edge is joined to the lower frame part, the pipe bracket is joined to the lower frame part with the lower frame part inserted into an opening of the pipe bracket in a manner that a spacing (d) in the width direction is provided between the outer side end portion of the lower frame part and an inner side surface of the second side frame part., wherein the spacing (d), the pipe bracket and the second frame part are configured such that the inner side surface of the second frame part contacts an upper part of the lower frame part upon deformation of the pipe bracket. a seat cushion on which a person is seated; and a seat back erected at a rear end of the seat cushion, the seat back being rotatably configured with respect to the seat cushion, wherein: a seat back frame forming a skeletal outline for the seat back includes an upper frame part formed of a pipe material, the upper frame part being placed on an upper part of the seat back frame, a lower frame part formed of a pipe material, the lower frame part being placed on a lower part of the seat back frame, a first side frame part formed of a plate member, the first side frame part being coupled between the upper frame part and the lower frame part on one side in a width direction of the vehicle seat, a second side frame part formed of a plate member, the second side frame part being coupled between the upper frame part and the lower frame part on the other side in the width direction, wherein an upper frame part formed of a pipe material, the upper frame part being placed on an upper part of the seat back frame, a lower frame part formed of a pipe material, the lower frame part being placed on a lower part of the seat back frame, a first side frame part formed of a plate member, the first side frame part being coupled between the upper frame part and the lower frame part on one side in a width direction of the vehicle seat, a second side frame part formed of a plate member, the second side frame part being coupled between the upper frame part and the lower frame part on the other side in the width direction, wherein the upper frame part, the lower frame part, the first side frame part, and the second side frame part are formed as separate components, and the upper frame part, the lower frame part, the first side frame part, and the second side frame part are joined to configure the seat back frame; the lower frame part is provided in a manner that a rotational axis about which the seat back is rotated with respect to the seat cushion is located on an inner side of the lower frame part; and a pipe bracket is provided between the lower frame part and the second side frame part, the pipe bracket joining the lower frame part to the second side frame part, characterized in that a lower panel is placed near the lower frame part; one end portion in the width direction of the lower panel is joined to the first side frame part, the other end portion in the width direction is joined to the second side frame part, and a lower edge is joined to the lower frame part, the pipe bracket is joined to the lower frame part with the lower frame part inserted into an opening of the pipe bracket in a manner that a spacing (d) in the width direction is provided between the outer side end portion of the lower frame part and an inner side surface of the second side frame part., wherein the spacing (d), the pipe bracket and the second frame part are configured such that the inner side surface of the second frame part contacts an upper part of the lower frame part upon deformation of the pipe bracket.