PURPOSE: To make possible the controlling of the quantity of welding heat input to quentity within a preset range by providing a welding heat input controller to a welding machine, obtaining the average quantity of welding heat input from the detected values of the voltage and current at the welding and welding length and controlling the same.

CONSTITUTION: In welding the materials 13 to be welded with a welding machine 10, the values of the welding current and voltage during welding work are momentarily detected with a welding current and welding voltage detector 1, and the products of the voltage and the current are operated by the device 1. The cumulative value thereof is inputted to a welding heat controller 3. At the same time, the length of the welding length instructing section actually measured with a welding length instructing device 2 is also inputted. The cumulated value of the produce of the voltage and current values is divided by the data from the welding length instructing device, and the average quantity of welding heat input per unit length within this section is calculated and is inputted to a recorder 9. When this value exceeds the control threshold of the correct average quantity of heat input, an alarm is emitted, whereby the welding voltage and current are controlled.