Pre-DECIGO consists of three spacecraft arranged in an equilateral trianglewith 100km arm lengths orbiting 2000km above the surface of the earth. It ishoped that the launch date will be in the late 2020s. Pre-DECIGO has one clear target: binary black holes (BBHs) like GW150914 andGW151226. Pre-DECIGO can detect $\sim 30M_\odot-30M_\odot$ BBH mergers up toredshift $z\sim 30$. The cumulative event rate is $\sim 1.8\times 10^{5}\,{\rmevents~yr^{-1}}$ in the Pop III origin model of BBHs like GW150914, and itsaturates at $z\sim 10$, while in the primordial BBH (PBBH) model, thecumulative event rate is $ \sim 3\times 10^{4}\,{\rm events~ yr^{-1}}$ at$z=30$ even if only $0.1\%$ of the dark matter consists of PBHs, and it isstill increasing at $z=30$. In the Pop I/II model of BBHs, the cumulative eventrate is $(3-10)\times10^{5}\,{\rm events~ yr^{-1}}$ and it saturates at $z \sim6$. We present the requirements on orbit accuracy, drag free techniques, laserpower, frequency stability, and interferometer test mass. For BBHs likeGW150914 at 1Gpc, SNR$\sim 90$ is achieved with the definition of Pre-DECIGO inthe $0.01-100$Hz band. Pre-DECIGO can measure the mass spectrum and the$z$-dependence of the merger rate to distinguish various models of BBHs likeGW150914. Pre-DECIGO can also predict the direction of BBHs at $z=0.1$ with an accuracyof $\sim 0.3\,{\rm deg}^2$ and a merging time accuracy of $\sim 1$s at about aday before the merger so that ground-based GW detectors further developed atthat time as well as electromagnetic follow-up observations can prepare for thedetection of merger in advance. For intermediate mass BBHs at a large redshift$z > 10$, the QNM frequency after the merger can be within the Pre-DECIGO bandso that the ringing tail can also be detectable to confirm the Einstein theoryof general relativity with SNR$\sim 35$. [abridged]
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