If the tensor-to-scalar ratio r of cosmological perturbations takes a large value r ∼ 0.1, which may be inferred from the recent BICEP2 result, we can hope to determine thermal history, in particular, the reheating temperature, TR, after inflation by space-based laser interferometers. It is shown that upgraded and upshifted versions of DECIGO may be able to determine TR if it lies in the …