A door trim installed in a vehicle, the door trim comprising: an upper board defining an upper portion of the door trim; a lower board including a body defining a lower portion of the door trim and a mounting flange extending upward from an upper edge of the body and opposed to the upper board; and a fastening structure (50, 60) including: a mounting boss protruding from the upper board or the mounting flange of the lower board, whichever disposed more interior, the mounting boss protruding toward an exterior of the vehicle; and a mounting hole formed in the upper board or the mounting flange of the lower board, whichever disposed more exterior, the mounting hole including: a through hole portion in which the mounting boss is fitted; and a slot portion extending from a front section of a hole edge of the through hole portion toward a front of the vehicle to communicate with the through hole portion wherein the upper board includes a front portion closer to the front of the vehicle and the rear portion closer to a rear of the vehicle, the mounting flange is opposed to a back face of the rear portion of the upper board, characterized in that the lower board further includes a rear edge portion disposed closer to the rear of the vehicle than the mounting flange so that at least a section of the rear edge portion is disposed behind a pillar of the vehicle. an upper board defining an upper portion of the door trim; a lower board including a body defining a lower portion of the door trim and a mounting flange extending upward from an upper edge of the body and opposed to the upper board; and a fastening structure (50, 60) including: a mounting boss protruding from the upper board or the mounting flange of the lower board, whichever disposed more interior, the mounting boss protruding toward an exterior of the vehicle; and a mounting hole formed in the upper board or the mounting flange of the lower board, whichever disposed more exterior, the mounting hole including: a through hole portion in which the mounting boss is fitted; and a slot portion extending from a front section of a hole edge of the through hole portion toward a front of the vehicle to communicate with the through hole portion wherein the upper board includes a front portion closer to the front of the vehicle and the rear portion closer to a rear of the vehicle, the mounting flange is opposed to a back face of the rear portion of the upper board, characterized in that the lower board further includes a rear edge portion disposed closer to the rear of the vehicle than the mounting flange so that at least a section of the rear edge portion is disposed behind a pillar of the vehicle. a mounting boss protruding from the upper board or the mounting flange of the lower board, whichever disposed more interior, the mounting boss protruding toward an exterior of the vehicle; and a mounting hole formed in the upper board or the mounting flange of the lower board, whichever disposed more exterior, the mounting hole including: a through hole portion in which the mounting boss is fitted; and a slot portion extending from a front section of a hole edge of the through hole portion toward a front of the vehicle to communicate with the through hole portion wherein a through hole portion in which the mounting boss is fitted; and a slot portion extending from a front section of a hole edge of the through hole portion toward a front of the vehicle to communicate with the through hole portion wherein the upper board includes a front portion closer to the front of the vehicle and the rear portion closer to a rear of the vehicle, the mounting flange is opposed to a back face of the rear portion of the upper board, characterized in that the lower board further includes a rear edge portion disposed closer to the rear of the vehicle than the mounting flange so that at least a section of the rear edge portion is disposed behind a pillar of the vehicle.