Article 2 of the Imperial House Law fixes the order in which the throne passes. It begins with kōchōshi, the emperor’s eldest son, and kōchōson, the eldest son’s eldest son. Then come the other descendants of the eldest son, and only after them the second son and his descendants.
The third paragraph adds a rule for these cases: Chōkei o saki ni shi, dōtōnai de wa, chō o saki ni suru. The senior line comes first, and within the same degree, the elder comes first.