Hideya,
The serial port sends one text character at a time. So you must convert your numeric value to a text or string representation and then send the string.
LabVIEW has several functions on the String palette for numeric conversions. You must decide how to represent the number as text, floating point or exponential notation, number of significant digits, and other formatting issues. These may be dependent on what will be done with the data after being sent.
Lynn