To provide a flat alkaline primary battery of low cost with high battery voltage which can endure usage for a long time and can notify in advance running out of a battery and has a high reliability.

The flat alkaline primary battery has a positive electrode can 2 and a negative electrode can 3. The negative electrode can 2 is inserted and engaged with a circular aperture 2a of the positive electrode can 2 from an aperture 3a side where a gasket 4 is installed, and the aperture 2a of the positive electrode can 2 is caulked and sealed toward the gasket 4, thereby a sealed space is formed between the positive electrode can and the negative electrode can through the gasket 4. A positive electrode mixture 5, a separator 6, and a negative electrode mixture 7 are housed in the sealed space, and the positive electrode mixture 5 is housed and arranged in the positive electrode can 2 side and the negative electrode mixture 7 is housed in the negative electrode can 3 side. An alkali electrolytic liquid is filled in the sealed space. Then, zinc or zinc alloy powder is made a negative electrode active material, and nickel oxy-hydroxide is made a positive electrode active material, and the mass of nickel oxy-hydroxide is made 3.3 times or more and 3.8 times or less against the zinc mass.