PURPOSE: To obtain a heat sink unit which has excellent heat dissipating property capable of effectively dissipating the heat without affecting the influence of the heat generated from a semiconductor relay mounted on the unit to the other.

CONSTITUTION: Since a heat sink unit 1 is formed at the center with a heat sink cylinder 1a and a plurality of mounting fins 1d for mounting a semiconductor relay 2 are radially formed in the prescribed slope on the lower side of the cylinder 1 at the independent partition wall 1e, the heat generated from the relay 2 is dissipated to the atmosphere through a vent cylinder 1c and a heat sink cylinder 1a at the upper center to improve the heat dissipation, a conical crest partition wall 1e is formed at the lower central end of the cylinder 1a, the holes of the opposed mounting fins are shielded to eliminate the influence of the discharged hot air to the other. Therefore, the heat dissipation of a plurality of semiconductor relays mounted on the lower surface side of the mounting fins can be effectively performed.