The heat pulses produced by cutting grains in a workpiece were measured using an infrared radiation pyrometer connected by means of an optical fiber. The results obtained were compared with those from a thermocouple to investigate the effect of differences in response speed on the output. The I.R.P., using an InAs cell as a detector which has a response time in the order of μs, can detect heat pulses with great accuracy and its signal trace versus time has many sharp peaks. The thermocouple formed by spot welding is inferior in response speed, and less accurate in registering heat pulses.