We studied the myocardial cell necrosis measuring cell lethality with various albumin concentrations against oxidative stress by extracellular photosensitization reaction using talaporfin sodium. We have proposed to induce the photosensitization reaction in interstitial space of myocardium with short drug-light interval to realize the immediate cell damage by myocardial cell membrane for tachyarrhythmia treatment. To understand the effect of the particular extracellular photosensitization reaction in interstitial space, we measured the myocardial cell lethality 2 hours after the photosensitization reaction by WST assay varying the talaporfin sodium concentration, radiant exposure, and wide albumin concentration of 0-15 mg/ml. The cell lethality was decreased with albumin concentration increasing over 85 % in binding ratio between talaporfin sodium and albumin. The cell lethality didnâ€™t change between 0-85% in binding ratio between talaporfin sodium and albumin. The calculated deposited energy to the talaporfin sodium solution was nearly constant of 8.4±1.6 J/well in average varying albumin concentration. We think the cell killing effect by the extracellular photosensitization reaction has a threshold between 85-100% in the binding ratio between talaporfin sodium and albumin in the case of the talaporfin sodium concentration of 40 μg/ml and radiant exposure of 0-40 J/cm2.