A spray gun for forming a resin layer made of low-density resin by spraying raw material liquid along with gas, comprising: a main body section into which the raw material liquid is introduced; and a nozzle section provided at a leading end of the main body section for mixing gas into the raw material liquid and ejecting the raw material liquid, characterized in that an internal space of the nozzle section includes: an introduction path into which the raw material liquid is introduced; an open portion in which a flow path is enlarged more than the introduction path for spouting the raw material liquid in a fine particle form; a reduced diameter portion in which a flow path is made narrower than the open portion for re-aggregating the raw material liquid in a fine particle form; and an ejection orifice portion which ejects the raw material liquid to the outside, the open portion has an enlarged diameter portion in which the diameter thereof is gradually enlarged toward a leading end direction from an outlet of the introduction path, a gas introduction hole which has an opening at the enlarged diameter portion and introduces the gas into the enlarged diameter portion, and a gas introduction portion which leads the gas fed from the outside to the gas introduction hole are formed in the nozzle section, a check valve is provided at the gas introduction portion, the inner diameter of the gas introduction hole is narrower than the inner diameter of the open portion, the inner diameter of the gas introduction portion is larger than the inner diameter of the gas introduction hole, and the gas introduction hole is inclined toward the leading end direction toward the inside of the nozzle section. a main body section into which the raw material liquid is introduced; and a nozzle section provided at a leading end of the main body section for mixing gas into the raw material liquid and ejecting the raw material liquid, characterized in that an internal space of the nozzle section includes: an introduction path into which the raw material liquid is introduced; an open portion in which a flow path is enlarged more than the introduction path for spouting the raw material liquid in a fine particle form; a reduced diameter portion in which a flow path is made narrower than the open portion for re-aggregating the raw material liquid in a fine particle form; and an ejection orifice portion which ejects the raw material liquid to the outside, the open portion has an enlarged diameter portion in which the diameter thereof is gradually enlarged toward a leading end direction from an outlet of the introduction path, a gas introduction hole which has an opening at the enlarged diameter portion and introduces the gas into the enlarged diameter portion, and a gas introduction portion which leads the gas fed from the outside to the gas introduction hole are formed in the nozzle section, a check valve is provided at the gas introduction portion, the inner diameter of the gas introduction hole is narrower than the inner diameter of the open portion, the inner diameter of the gas introduction portion is larger than the inner diameter of the gas introduction hole, and the gas introduction hole is inclined toward the leading end direction toward the inside of the nozzle section. an introduction path into which the raw material liquid is introduced; an open portion in which a flow path is enlarged more than the introduction path for spouting the raw material liquid in a fine particle form; a reduced diameter portion in which a flow path is made narrower than the open portion for re-aggregating the raw material liquid in a fine particle form; and an ejection orifice portion which ejects the raw material liquid to the outside, the open portion has an enlarged diameter portion in which the diameter thereof is gradually enlarged toward a leading end direction from an outlet of the introduction path, a gas introduction hole which has an opening at the enlarged diameter portion and introduces the gas into the enlarged diameter portion, and a gas introduction portion which leads the gas fed from the outside to the gas introduction hole are formed in the nozzle section, a check valve is provided at the gas introduction portion, the inner diameter of the gas introduction hole is narrower than the inner diameter of the open portion, the inner diameter of the gas introduction portion is larger than the inner diameter of the gas introduction hole, and the gas introduction hole is inclined toward the leading end direction toward the inside of the nozzle section.