The new technique of measuring frequency by optical lattice clocks now approaches to the relative precision of $(Delta f/f)=O(10^{-18})$. We propose to place such precise clocks in space and to use Doppler tracking method for detecting low-frequency gravitational wave below 1 Hz. Our idea is to locate three spacecrafts at one A.U. distance (say at L1, L4 & L5 of the Sun-Earth orbit), and apply the Doppler tracking method by communicating the time each other. Applying the current available technologies, we obtain the sensitivity for gravitational wave with three or four-order improvement ($h_{rm n}sim 10^{-17}$ or $10^{-18}$ level in $10^{-5}$Hz -- $1$ Hz) than that of Cassini spacecraft in 2001. This sensitivity enables us to observe black-hole mergers of their mass greater than $10^5 M_odot$ in the cosmological scale. Based on the hierarchical growth model of black-holes in galaxies, we estimate the event rate of detection will be 20-50 a year. We nickname INO (Interplanetary Network of Optical Lattice Clocks) for this system, named after Tadataka Ino (1745--1818), a Japanese astronomer, cartographer, and geodesist.
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