Gas sensor element and gas sensor

A gas sensor element comprising:

• an element substrate having a surface in which a gas inlet is open, and including an internal space into which a measurement target gas is introduced from the gas inlet;
• a leading end protection layer covering at least a face of the element substrate in which the gas inlet is open;
• a measurement electrode provided in the internal space;
• and
• a porous diffusion layer located at a position that is upstream of the measurement electrode in a flow direction of the measurement target gas and where a distance to the measurement electrode is 0.15 mm or less,
• wherein the porous diffusion layer has a porosity that is 5% or more and 25% or less and is lower than a porosity of the leading end protection layer, and
• the porous diffusion layer has a face orthogonal to the flow direction of the measurement target gas, the face accounting for 70% or more of a cross-section of a flow path of the measurement target gas, the cross-section being orthogonal to the flow direction of the measurement target gas,
• the gas sensor element further comprising:
• a diffusion control portion configured to apply predetermined diffusion resistance to the measurement target gas in the internal space,
• wherein the diffusion control portion has a porosity lower than the porosity of the porous diffusion layer, and is located upstream of measurement electrode in the flow direction of the measurement target gas,
• the flow path has at least one face defined by the diffusion control portion, and
• the porous diffusion layer is in contact with an upper or lower surface of the diffusion control portion in a direction orthogonal to the flow direction of the measurement target gas.