To provide a crack depth measurement method for correctly measuring a crack depth formed from a surface of a hardened cement object to the inside.

In the crack depth measurement method, the crack depth is calculated by the first formula: d=-α×λ×InF. In the first formula, the d is the crack depth (mm), and the λ is a primary wavelength (mm). The F is an amplitude ratio obtained by dividing the second minimum negative amplitude fluctuated toward the negative side right after a surface wave is initiated among the second surface waves propagated on a surface of the hardened cement object and detected in a second sensor 13 by the first minimum negative amplitude fluctuated toward the negative side right after the surface wave is initiated among the first surface waves propagated on the surface of the hardened cement object and detected in a first sensor 12, and the α is a constant (0.381) calculated from an approximate expression: y=αebx derived from a correlation diagram for indicating the (F) along a vertical axis, and a ratio of the crack depth (d)/the primary wavelength (λ) along a horizontal axis.