1. A tape printing device for printing an input text on tape, comprising: first judgement means for judging whether a number of lines of characters of the input text is not less than two, and whether a character-size entrusting mode, in which a character size corresponding to each line is automatically determined, is selected; length information generating means for assigning a same character size to all of the lines and generating length information representing a length of each line to which the same character size is assigned when it is judged by the first judgement means that the character-size entrusting mode is selected and the number of lines of characters of the input text is not less than two; means for tentatively determining and assigning a different character size to at least one line other than a longest line according to the length information so as to make the length of the at least one other line, which is calculated on the basis of the character size assigned thereto, equal to the length of the longest line; second judgement means for judging whether a total sum of transverse sizes of all of the lines exceeds a predetermined limited size, the total sum being calculated on the basis of the character size assigned by the length information generating means to the longest line and the character size assigned to the at least one other line and determined tentatively by the tentatively determining means; means for determining the character size assigned by the length information generating means to the longest line and the character size assigned to the at least one other line by the tentatively determining means as the character size attributes of the lines when it is judged by the second judgement means that the total sum of the transverse sizes of all of the lines does not exceed the predetermined limited size; and means for determining the character size attributes of the lines by multiplying the character sizes assigned by the length information generating means to the longest line and assigned to the at least one other line by the tentatively determining means by a coefficient determined according to the limited size and the total sum of the transverse sizes of the lines, when it is judged by the second judgement means that the total sum of the transverse sizes of all of the fines exceeds the predetermined limited size; and print means for printing the input text according to the character-size attribute of each line.