PROBLEM TO BE SOLVED: To provide a gas sensor capable of stably measuring the concentration of ammonia gas.SOLUTION: A gas sensor includes: a first chamber 101C1 having a breath inlet port 102A; a second chamber 101C2; a gas sensitive element 130 provided in the inner part of the second chamber 101C2 and a resistance value thereof changes according to the concentration of a predetermined gas included in a breath; a partition portion 120 provided between the first chamber and the second chamber and switching between a connected state and a blocked state of the first chamber and the second chamber; and a measurement part 150 for measuring the concentration of the gas based on a temporal change degree of a resistance value of the gas sensitive element 130 in a state that the breath introduced into the first chamber in a state that the first chamber and the second chamber are blocked by the partition part 120 is in a state that the first chamber and the second chamber are connected by the partition part 120 and introduced into the second chamber.SELECTED DRAWING: Figure 1【課題】アンモニアガスの濃度を安定的に測定できるガスセンサを提供する。【解決手段】ガスセンサは、呼気導入口102Aを有する第1チャンバ101C1と、第2チャンバ101C2と、第2チャンバ101C2の内部に設けられ、呼気に含まれる所定のガスの濃度に応じて抵抗値が変化するガス感応素子130と、第1チャンバと第2チャンバとの間に設けられ、第1チャンバと第2チャンバとが連通された状態と遮断された状態とを切り替える仕切部120と、第1チャンバと第2チャンバとが仕切部120によって遮断された状態で第1チャンバに導入された呼気が、仕切部120によって第1チャンバと第2チャンバとが連通されて第2チャンバに導入された状態で、ガス感応素子130の抵抗値の時間的変化度合に基づいて、ガスの濃度を計測する計測部150とを含む。【選択図】図1