PURPOSE: To preserve energy and to reduce fuel by providing a solar heat col- lector in a circulation passage to a shipboard heater utilizing marine diesel engine coolant heat and constructing to auxiliarily heat coolant.

CONSTITUTION: A bypass valve 19 is provided in the fresh water circulation passage for heating a low temperature heater 12, and both ends of a solar heat collector 18 are connected to both ends of a bypass valve 19. During normal voyage, the heater 12 is heated by the first stage fresh water heater 11 with coolant of a main diesel engine 1 as a heat source, and necessary heat is obtained by the collector 18 mainly during low speed or mooring. Further, when heat is insufficient, the steam of an auxiliary boiler 3 is utilized to heat with the second stage fresh water heater 14.