We have measured the susceptibility and low-temperature magnetization curve of the novel organic tetraradical crystal BIP-TENO. The susceptibility data indicate that double-spin chains of ferromagnetic dimers are formed in the crystal and the spin system is regarded as an S=1 antiferromagnetic two-leg ladder. The magnetization is nearly zero up to 100kOe and the spin gap is closed at 116kOe. Above 120kOe, the magnetization increases and a plateau corresponding to a quarter of the saturation magnetization appears at 448kOe.