A transmission characteristic value determination method of a laser marking a battery cell whose appearance is formed of a heat shrinkable tube , the method comprising the following order of steps: first a temporary setting step (S110; S210) of setting a laser frequency to 20 kHz to 100 kHz, a marking speed to 50 mm/sec to 5000 mm/sec, and a focal distance of a laser to 10 mm to 300 mm which are predetermined temporary setting values, wherein the focal length is a value at which the size of the readout symbol is adjusted, and the frequency and the marking speed are values mutually set to a value at which the depth of the readout symbol is adjusted; then marking (S120; S220) a letter on the heat shrinkable tube by transmitting the laser beam using temporary setting values set in the temporary setting step (S110; S210); then a heating step (S130; S230) of applying heat to the heat shrinkable tube leading to a shrinkage of the tube ; and then a font size checking step (S140) of determining, after the heating step (S130; S230), whether a font size of the letter marked on the heat shrinkable tube is greater than or equal to a preset font size, the method further comprising: if the font size of the letter checked in the font size checking step (S140) is equal to or greater than a preset font size, a transmission characteristic value storing step of storing the frequency, the marking speed and the focal distance set in the temporary setting step (S110; S210) in a separate storage space, and if the font size of the letter is less than the preset font size, the marking step (S120; S220) being performed again by correcting the frequency, the marking speed and the focal distance. first a temporary setting step (S110; S210) of setting a laser frequency to 20 kHz to 100 kHz, a marking speed to 50 mm/sec to 5000 mm/sec, and a focal distance of a laser to 10 mm to 300 mm which are predetermined temporary setting values, wherein the focal length is a value at which the size of the readout symbol is adjusted, and the frequency and the marking speed are values mutually set to a value at which the depth of the readout symbol is adjusted; then marking (S120; S220) a letter on the heat shrinkable tube by transmitting the laser beam using temporary setting values set in the temporary setting step (S110; S210); then a heating step (S130; S230) of applying heat to the heat shrinkable tube leading to a shrinkage of the tube ; and then a font size checking step (S140) of determining, after the heating step (S130; S230), whether a font size of the letter marked on the heat shrinkable tube is greater than or equal to a preset font size, the method further comprising: if the font size of the letter checked in the font size checking step (S140) is equal to or greater than a preset font size, a transmission characteristic value storing step of storing the frequency, the marking speed and the focal distance set in the temporary setting step (S110; S210) in a separate storage space, and if the font size of the letter is less than the preset font size, the marking step (S120; S220) being performed again by correcting the frequency, the marking speed and the focal distance. <EOS>