N103B is a Type Ia supernova remnant (SNR) in the Large Magellanic Cloud (LMC). We carried out new 12CO(J = 3-2) and 12CO(J = 1-0) observations using ASTE and ALMA. We have confirmed the existence of a giant molecular cloud at VLSR ∼ 245 km s−1 toward the southeast of the SNR using ASTE 12CO(J = 3-2) data at an angular resolution of ∼25″ (∼6 pc in the LMC). Using the ALMA 12CO(J = 1-0) data, we have spatially resolved CO clouds along the southeastern edge of the SNR with an angular resolution of ∼1 8 (∼0.4 pc in the LMC). The molecular clouds show an expanding gas motion in the position-velocity diagram with an expansion velocity of ∼5 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.