1. A control method for an arc welding process, comprising the steps of: (a) obtaining optical information from light produced from an electric arc during an arc welding process; (b) comparing the obtained optical information with reference information; (c) adjusting welding conditions on the basis of a result of comparison made between the obtained optical information and the reference information; (d) carrying out the arc welding process under the adjusted welding conditions; and (e) repeating the preceding processes (a) to (d) in the order named to thereby perform the arc welding process.