A steel pipe material weld zone heating apparatus that, in making a tubular steel pipe material while bending a moving steel plate into cylindrical shape, uses a variable frequency alternating current power supply, whose frequency is variable within a predetermined frequency range, to generate joule heat by passing alternating current near end faces of the steel pipe material to continuously melt and pressure-weld an end face weld zone of the steel pipe material, which steel pipe material weld zone heating apparatus comprises: first imaging means installed opposite an end face weld zone of the steel pipe material that detects self-emitted light of the weld zone and outputs a brightness image of the end face weld zone; a weld zone temperature distribution computation means that performs image processing based on the brightness image and applies emitted light temperature measurement to output a measured value of the weld zone two-dimensional temperature distribution whose axes lie in the weld zone thickness direction of the weld zone and the steel material longitudinal direction; second imaging means that images the weld zone of the steel pipe material from above and outputs a plane image of the molten steel material immediately after pressure-welding the steel pipe material; and weld zone melt shape measurement means that ascertains the melt shape based on the plane image and outputs a molten zone width value; welding current waveform measurement means that measures the current waveform of the alternating current; and welding frequency fluctuation measurement means that, based on a current waveform output by the welding current waveform measurement means, ascertains and outputs a welding frequency time-dependent fluctuation value that is the time-dependent fluctuation amplitude of the current waveform frequency; heating control means that, by use of a criterion predefined from the dimensions and electromagnetic properties of the steel material for evaluating the relationship between alternating current frequency and plate-thickness direction temperature distribution, determines the frequency of the alternating current based on the plate-thickness direction temperature distribution; and a variable frequency magnetic energy recovery type alternating current power supply for passing through the steel pipe material alternating current of the frequency determined by the heating control means, wherein the heating control means, by use of a predefined criterion for evaluating the molten zone width and the relationship between the alternating current frequency and the temperature rise in the plate thickness direction, determines the frequency of the alternating current based on the molten zone width value and the plate-thickness direction temperature distribution, and further compares the welding frequency time-dependent fluctuation value with a predetermined allowable value and determines the frequency of the alternating current. first imaging means installed opposite an end face weld zone of the steel pipe material that detects self-emitted light of the weld zone and outputs a brightness image of the end face weld zone; a weld zone temperature distribution computation means that performs image processing based on the brightness image and applies emitted light temperature measurement to output a measured value of the weld zone two-dimensional temperature distribution whose axes lie in the weld zone thickness direction of the weld zone and the steel material longitudinal direction; second imaging means that images the weld zone of the steel pipe material from above and outputs a plane image of the molten steel material immediately after pressure-welding the steel pipe material; and weld zone melt shape measurement means that ascertains the melt shape based on the plane image and outputs a molten zone width value; welding current waveform measurement means that measures the current waveform of the alternating current; and welding frequency fluctuation measurement means that, based on a current waveform output by the welding current waveform measurement means, ascertains and outputs a welding frequency time-dependent fluctuation value that is the time-dependent fluctuation amplitude of the current waveform frequency; heating control means that, by use of a criterion predefined from the dimensions and electromagnetic properties of the steel material for evaluating the relationship between alternating current frequency and plate-thickness direction temperature distribution, determines the frequency of the alternating current based on the plate-thickness direction temperature distribution; and a variable frequency magnetic energy recovery type alternating current power supply for passing through the steel pipe material alternating current of the frequency determined by the heating control means, wherein the heating control means, by use of a predefined criterion for evaluating the molten zone width and the relationship between the alternating current frequency and the temperature rise in the plate thickness direction, determines the frequency of the alternating current based on the molten zone width value and the plate-thickness direction temperature distribution, and further compares the welding frequency time-dependent fluctuation value with a predetermined allowable value and determines the frequency of the alternating current.