A tape cassette intended to be inserted into a tape printer having at least two guide shafts (100, 120) standing upward in a cassette housing portion of the tape printer, said tape cassette comprising: a generally rectangular box-like housing having a top wall, a bottom wall, and a side wall defining a periphery of the housing; and at least one tape (55, 57, 58, 59) wound and mounted within the housing in a tape containing area defined within the periphery; and an ink ribbon wound and mounted within the housing to be used for printing on a print medium tape (55, 57, 59), the print medium tape (55, 57, 59) being one of the at least one tape (55, 57, 58, 59) characterized by a pair of cavities (64 and 47) including a first cavity and a second cavity extending from the bottom wall and disposed between the tape containing area and the periphery at opposite ends of a diagonal of the generally rectangular box-like housing, the diagonal connecting a first corner portion and a second corner portion of the generally rectangular box-like housing, said cavities (64 and 47) being formed as through-holes that extend through the top wall and the bottom wall in a vertical direction of the housing, and said cavities (64 and 47) being suitable to have said at least two guide shafts (100, 120) respectively inserted into said cavities (64 and 47) so as to guide the tape cassette into the cassette housing portion of the tape printer and a center of gravity of one (55, 57, 58) of the at least one tape (55, 57, 58, 59) is located in a first area, the first area being one of two areas (30C and 30D) formed by dividing the housing with respect to a straight line connecting the pair of cavities (64 and 47); and a center of gravity of the ink ribbon is located in a second area, the second area being one of the two areas (30C and 30D) other than the first area. a generally rectangular box-like housing having a top wall, a bottom wall, and a side wall defining a periphery of the housing; and at least one tape (55, 57, 58, 59) wound and mounted within the housing in a tape containing area defined within the periphery; and an ink ribbon wound and mounted within the housing to be used for printing on a print medium tape (55, 57, 59), the print medium tape (55, 57, 59) being one of the at least one tape (55, 57, 58, 59) characterized by a pair of cavities (64 and 47) including a first cavity and a second cavity extending from the bottom wall and disposed between the tape containing area and the periphery at opposite ends of a diagonal of the generally rectangular box-like housing, the diagonal connecting a first corner portion and a second corner portion of the generally rectangular box-like housing, said cavities (64 and 47) being formed as through-holes that extend through the top wall and the bottom wall in a vertical direction of the housing, and said cavities (64 and 47) being suitable to have said at least two guide shafts (100, 120) respectively inserted into said cavities (64 and 47) so as to guide the tape cassette into the cassette housing portion of the tape printer and a center of gravity of one (55, 57, 58) of the at least one tape (55, 57, 58, 59) is located in a first area, the first area being one of two areas (30C and 30D) formed by dividing the housing with respect to a straight line connecting the pair of cavities (64 and 47); and a center of gravity of the ink ribbon is located in a second area, the second area being one of the two areas (30C and 30D) other than the first area.