1. An input supporting method of a variable name in programming of an PLC in which a computer supports selecting and inputting a variable name from a variable table, in which a variable name scheduled to be used in a user program is registered in advance with an attribute thereof, to specify a variable name that becomes an operand of one command configuring the user program of the PLC, the method comprising: a first step of obtaining a state one character of a character string configuring the variable name to become the operand of the one command is inputtable; a second step of extracting, from the variable table, all variable names that forward match with a character string confirmed up to now and the character string including the one character when the one character is inputted; a third step of completing specification of the operand of the one command by confirming the variable name as the operand in a case where the number of the extracted variable names is one; and a fourth step of returning to the first step with the character string of the portion forward matching with each other as a confirmed character string on all extracted variable names in a case where the number of the extracted variable names is more than one.