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We report improved frequency measurement of a single 40Ca+ optical clock towards the 10-15 uncertainty level. In contrast with our previous measurement, in which the accuracy was limited by the magnetic field fluctuation, the use of a magnetic shield significantly reduced the observed linewidth of its 2S1/2-2D5/2 clock transition. From frequency comparison between the 40Ca+ clock and an 87Sr lattice clock, an Allan deviation of 7 × 10-16 at an averaging time of one thousand seconds was measured. We now measure the absolute frequency of the 40Ca+ clock transition with respect to the SI definition of the second.