A monitoring apparatus for an electric resistance welding operation that forms a strip-like steel sheet continuously into a cylindrical shape with a group of rollers while the steel sheet is being transferred, and heats and melts both of circumferential ends of the formed steel sheet converging in a V shape, butts against each other, the monitoring apparatus comprising: an image obtaining unit that takes a photograph from at least either one of an external surface side and an internal surface side of the steel sheet when the steel sheet is formed into the cylindrical shape, to thereby obtain an image of a region including a Vee convergence section, which is an area where both of the circumferential ends converge in the V shape; either one of a contact point detection unit that, based on the image obtained by the image obtaining unit, detects a contact point (V 2) between both of the circumferential ends of the formed steel sheet converging in the V shape, and a Vee convergence point detection unit that, based on the image obtained by the image obtaining unit, detects a Vee convergence point, which is a geometrical convergence point between both of the circumferential ends of the formed steel sheet converging in the V shape; and characterized by a temperature measurement region setting unit that, based on either one of a position of the contact point detected by the contact point detection unit and a position of the Vee convergence point detected by the Vee convergence point detection unit, sets a temperature measurement region including a weld zone in which a fusion zone in an interior of a sheet thickness of the steel sheet begins to be discharged to a surface of the steel sheet; a luminance level computation unit that calculates a luminance level of the temperature measurement region having been set by the temperature measurement region setting unit; a temperature conversion unit that, based on a preset temperature conversion data, converts the luminance level of the temperature measurement region calculated by the luminance level computation unit to a temperature of the temperature measurement region; and a determination unit that determines whether a temperature of the temperature measurement region is more than or equal to a predetermined lower limit value. an image obtaining unit that takes a photograph from at least either one of an external surface side and an internal surface side of the steel sheet when the steel sheet is formed into the cylindrical shape, to thereby obtain an image of a region including a Vee convergence section, which is an area where both of the circumferential ends converge in the V shape; either one of a contact point detection unit that, based on the image obtained by the image obtaining unit, detects a contact point (V 2) between both of the circumferential ends of the formed steel sheet converging in the V shape, and a Vee convergence point detection unit that, based on the image obtained by the image obtaining unit, detects a Vee convergence point, which is a geometrical convergence point between both of the circumferential ends of the formed steel sheet converging in the V shape; and characterized by a temperature measurement region setting unit that, based on either one of a position of the contact point detected by the contact point detection unit and a position of the Vee convergence point detected by the Vee convergence point detection unit, sets a temperature measurement region including a weld zone in which a fusion zone in an interior of a sheet thickness of the steel sheet begins to be discharged to a surface of the steel sheet; a luminance level computation unit that calculates a luminance level of the temperature measurement region having been set by the temperature measurement region setting unit; a temperature conversion unit that, based on a preset temperature conversion data, converts the luminance level of the temperature measurement region calculated by the luminance level computation unit to a temperature of the temperature measurement region; and a determination unit that determines whether a temperature of the temperature measurement region is more than or equal to a predetermined lower limit value.