Inputting mathematical expressions with a mouse and a keyboard is a troublesome task. Thus, a number of mathematical expression recognition systems capable of recognizing handwritten mathematical expressions to input them into computers have been proposed. Even with these systems, however, structure recognition of mathematical expressions is still difficult. This paper presents MathBox, a new pen-based interface for inputting mathematical expressions into computers. MathBox interactively shows "boxes" in which the user can write one symbol. The boxes are shown along with the user's writing. For example, when the user writes 'x,' the boxes for a power and an index of 'x' and for the next symbol are shown. When the user inputs a fraction line, boxes for the numerator, denominator, and the next symbol are shown. MathBox skips recognizing the structures of expressions, which enables users to write mathematical expressions with practical accuracy.