To provide a boiler system where liquid fuel is normally sprayed from a liquid spray nozzle of a fuel switch burner.

A boiler system includes: a boiler body 2; a liquid burner equipped with a liquid spray nozzle; a liquid fuel supply line L1; a liquid fuel valve V1; a gas burner; a gas fuel supply line L2; a gas fuel valve V5; an air supply unit having a first air supply line L3 and a second air supply line L4 of which one end is connected to the first air supply line L3 and the other end is connected to downstream sides of liquid fuel valves V2, V3 in the liquid fuel supply line L1; an air valve V7; and a control unit 10 for controlling opening/closing of the liquid fuel valve V1 and the gas fuel valve V5 to switch fuel between liquid fuel and gas fuel. The control unit 10 closes the air valve V7 when the liquid fuel is to be burned in a combustion chamber, and closes the air valve V7 to supply air to the liquid spray nozzle when the fuel is switched from liquid fuel to gas fuel.