To provide a simple estimation method for a bentonite mixing ratio capable of accurately estimating the bentonite mixing ratio in bentonite-mixed earth regardless of properties and particle size of the bentonite-mixed earth.

This estimation method comprises a process wherein a concentration of a specific ion contained in a bentonite-mixed earth solution formed by mixing water with earth having a known bentonite mixing ratio using the same earth as bentonite-mixing earth having an unknown bentonite mixing ratio at a specific ratio is measured to measure the correlation between the concentration of the specific ion and a bentonite mixing ratio, a process wherein the concentration of the specific ion contained in a bentonite-mixed earth solution formed by mixing water with the bentonite-mixed earth having the unknown bentonite mixing ratio at the specific ratio is measured, and a process wherein the bentonite mixing ratio of the bentonite-mixed earth having the unknown bentonite mixing ratio is estimated on the basis of the results of the preceding processes.