A hot water supply system comprising: a hot water storage tank configured to store hot water; a hot water delivery unit configured to supply the hot water from the hot water storage tank to a usage side; a heat pump heating unit configured to boil the hot water in the hot water storage tank; and an auxiliary heating unit configured to heat the hot water supplied from the hot water storage tank to the hot water delivery unit, wherein the hot water supply system is configured to perform a heat pump isolated operation that boils the hot water in the hot water storage tank using the heat pump heating unit, the heat pump isolated operation includes boiling in a first mode and boiling in a second mode having heating capabilities different from each other, and the heating capability of the boiling in the first mode is higher than the heating capability of the boiling in the second mode, and heat pump efficiency of the boiling in the first mode is lower than heat pump efficiency of the boiling in the second mode, characterized in that: the hot water supply system is further configured to perform an auxiliary heating combined operation that simultaneously performs boiling of the hot water in the hot water storage tank in the second mode and heating of the hot water using the auxiliary heating unit when an amount of heat is insufficient only by the boiling of the hot water in the hot water storage tank using the heat pump heating unit; and the boiling in the first mode is performed in a predetermined condition where a shortage of the amount of heat is likely to be caused by the boiling in the second mode in a state where a large amount of delivery of the hot water is estimated or a large amount of the hot water is actually delivered. a hot water storage tank configured to store hot water; a hot water delivery unit configured to supply the hot water from the hot water storage tank to a usage side; a heat pump heating unit configured to boil the hot water in the hot water storage tank; and an auxiliary heating unit configured to heat the hot water supplied from the hot water storage tank to the hot water delivery unit, wherein the hot water supply system is configured to perform a heat pump isolated operation that boils the hot water in the hot water storage tank using the heat pump heating unit, the heat pump isolated operation includes boiling in a first mode and boiling in a second mode having heating capabilities different from each other, and the heating capability of the boiling in the first mode is higher than the heating capability of the boiling in the second mode, and heat pump efficiency of the boiling in the first mode is lower than heat pump efficiency of the boiling in the second mode, characterized in that: the hot water supply system is further configured to perform an auxiliary heating combined operation that simultaneously performs boiling of the hot water in the hot water storage tank in the second mode and heating of the hot water using the auxiliary heating unit when an amount of heat is insufficient only by the boiling of the hot water in the hot water storage tank using the heat pump heating unit; and the boiling in the first mode is performed in a predetermined condition where a shortage of the amount of heat is likely to be caused by the boiling in the second mode in a state where a large amount of delivery of the hot water is estimated or a large amount of the hot water is actually delivered. the hot water supply system is further configured to perform an auxiliary heating combined operation that simultaneously performs boiling of the hot water in the hot water storage tank in the second mode and heating of the hot water using the auxiliary heating unit when an amount of heat is insufficient only by the boiling of the hot water in the hot water storage tank using the heat pump heating unit; and the boiling in the first mode is performed in a predetermined condition where a shortage of the amount of heat is likely to be caused by the boiling in the second mode in a state where a large amount of delivery of the hot water is estimated or a large amount of the hot water is actually delivered.