A tape cassette comprising: a cassette case that has a top surface, a bottom surface, a front surface, and a pair of side surfaces; a tape roll (551, 571, 591) that is a wound tape (55, 57, 59) that is a printing medium and that is housed in the cassette case; an arm portion that includes a part of the front surface and that guides the tape toward a discharge opening along a section of a predetermined feed path, the section extending in parallel with the front surface; a long hole that extends in a left-right direction of the cassette case at the rear of and adjacent to the arm portion, that penetrates the cassette case in an up-down direction, and that is formed straddling a center position of the cassette case in the left-right direction, the long hole having a first end portion and a second end portion, the first end portion being an end portion located on a downstream side in a feed direction of the tape that is fed in the arm portion, and the second end portion being located on an upstream side in the feed direction; and characterized by a flat surface portion that is provided on an edge of the long hole and that is disposed closer to the first end portion of the long hole than to the second end portion of the long hole, the flat surface portion comprising a flat surface, the flat surface portion being recessed from the bottom surface in the up-down direction, the flat surface being disposed between the bottom surface and the top surface in the up-down direction, wherein a first distance, which is a length of the long hole in the left-right direction, is larger than a second distance, which is a length from the second end portion of the long hole to an end portion of the tape cassette that is located on the upstream side in the feed direction, and a position of a center of gravity of the tape cassette is located in a specific area, which is an area whose both end positions in the left-right direction are the same as the first end portion and the second end portion of the long hole, and which extends in a front-rear direction of the tape cassette. a cassette case that has a top surface, a bottom surface, a front surface, and a pair of side surfaces; a tape roll (551, 571, 591) that is a wound tape (55, 57, 59) that is a printing medium and that is housed in the cassette case; an arm portion that includes a part of the front surface and that guides the tape toward a discharge opening along a section of a predetermined feed path, the section extending in parallel with the front surface; a long hole that extends in a left-right direction of the cassette case at the rear of and adjacent to the arm portion, that penetrates the cassette case in an up-down direction, and that is formed straddling a center position of the cassette case in the left-right direction, the long hole having a first end portion and a second end portion, the first end portion being an end portion located on a downstream side in a feed direction of the tape that is fed in the arm portion, and the second end portion being located on an upstream side in the feed direction; and characterized by a flat surface portion that is provided on an edge of the long hole and that is disposed closer to the first end portion of the long hole than to the second end portion of the long hole, the flat surface portion comprising a flat surface, the flat surface portion being recessed from the bottom surface in the up-down direction, the flat surface being disposed between the bottom surface and the top surface in the up-down direction, wherein a first distance, which is a length of the long hole in the left-right direction, is larger than a second distance, which is a length from the second end portion of the long hole to an end portion of the tape cassette that is located on the upstream side in the feed direction, and a position of a center of gravity of the tape cassette is located in a specific area, which is an area whose both end positions in the left-right direction are the same as the first end portion and the second end portion of the long hole, and which extends in a front-rear direction of the tape cassette.