N103B is a Type Ia supernova remnant (SNR) in the Large Magellanic Cloud (LMC). We carried out new $^{12}$CO($J$ = 3-2) and $^{12}$CO($J$ = 1-0) observations using ASTE and ALMA. We have confirmed the existence of a giant molecular cloud (GMC) at $V_mathrm{LSR}$ $sim$245 km s$^{-1}$ towards the southeast of the SNR using ASTE $^{12}$CO($J$ = 3-2) data at an angular resolution of $sim$25$$ ($sim$6 pc in the LMC). Using the ALMA $^{12}$CO($J$ = 1-0) data, we have spatially resolved CO clouds along the southeastern edge of the SNR with an angular resolution of $sim$1.8$$ ($sim$0.4 pc in the LMC). The molecular clouds show an expanding gas motion in the position-velocity diagram with an expansion velocity of $sim5$ km s$^{-1}$. The spatial extent of the expanding shell is roughly similar to that of the SNR. We also find tiny molecular clumps in the directions of optical nebula knots. We present a possible scenario that N103B exploded in the wind-bubble formed by the accretion winds from the progenitor system, and is now interacting with the dense gas wall. This is consistent with a single-degenerate scenario.
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