A manufacturing method of a current transformer core, comprising: the step of providing a core element formed by winding or layering a Fe-based amorphous alloy ribbon whose thickness is not more than 15 μm and which can be converted into nanocrystals; a longitudinal-field heat treatment step which includes heating the core element to not less than a crystallizing temperature and within the range of 510°C to 600°C for 5 minutes to 24 hours in the presence of a magnetic field of not less than 100 A/m applied in a magnetic path direction of the core element, thereby forming a core; and a transverse-field heat treatment step which includes, after the longitudinal-field heat treatment step, heating the core to a temperature less than the crystallizing temperature and at not less than 200°C for 20 minutes to 120 minutes in the presence of a magnetic field of not less than 80 kA/m applied in a direction perpendicular to the magnetic path direction of the core, thereby forming a current transformer core, wherein μr is adjusted by the transverse-field heat treatment step to a value between 0.4×μr(max) and 0.9×μr(max) and μr ≥4×10 5 where μr(max) is μr achieved by the longitudinal-field heat treatment step, and μr is an amplitude magnetic permeability of the core measured at a temperature T (°C) in the presence of an AC magnetic field of frequency f=50 Hz and amplitude H=1.0 A/m, wherein the composition of the Fe-based nanocrystalline alloy is represented by the following formula: (Fe 1-a Ma) 100-x-y-z-α-β-γ Cu x Si y B z M' α M" β X γ (atom%) where M is Co and/or Ni, M' is at least one element selected from the group consisting of Nb, W, Ta, Zr, Hf, Ti and Mo, M" is at least one element selected from the group consisting of V, Cr, Mn, Al, platinum-group elements, Sc, Y, rare earth elements, Au, Zn, Sn and Re, X is at least one element selected from the group consisting of C, Ge, P, Ga, Sb, In, As and Be, and the factors that define the composition, a, x, y, z, α, β and γ, respectively meet the following conditions: 0≤a<0.5 0.1≤x≤3 10≤y≤20 5≤z≤10 0.1≤α≤5 0≤β≤10 0≤γ≤10, and wherein the Fe-based amorphous alloy ribbon is formed from a molten alloy by a liquid quenching method. the step of providing a core element formed by winding or layering a Fe-based amorphous alloy ribbon whose thickness is not more than 15 μm and which can be converted into nanocrystals; a longitudinal-field heat treatment step which includes heating the core element to not less than a crystallizing temperature and within the range of 510°C to 600°C for 5 minutes to 24 hours in the presence of a magnetic field of not less than 100 A/m applied in a magnetic path direction of the core element, thereby forming a core; and a transverse-field heat treatment step which includes, after the longitudinal-field heat treatment step, heating the core to a temperature less than the crystallizing temperature and at not less than 200°C for 20 minutes to 120 minutes in the presence of a magnetic field of not less than 80 kA/m applied in a direction perpendicular to the magnetic path direction of the core, thereby forming a current transformer core, wherein μr is adjusted by the transverse-field heat treatment step to a value between 0.4×μr(max) and 0.9×μr(max) and μr ≥4×10 5 where μr(max) is μr achieved by the longitudinal-field heat treatment step, and μr is an amplitude magnetic permeability of the core measured at a temperature T (°C) in the presence of an AC magnetic field of frequency f=50 Hz and amplitude H=1.0 A/m, wherein the composition of the Fe-based nanocrystalline alloy is represented by the following formula: (Fe 1-a Ma) 100-x-y-z-α-β-γ Cu x Si y B z M' α M" β X γ (atom%) where M is Co and/or Ni, M' is at least one element selected from the group consisting of Nb, W, Ta, Zr, Hf, Ti and Mo, M" is at least one element selected from the group consisting of V, Cr, Mn, Al, platinum-group elements, Sc, Y, rare earth elements, Au, Zn, Sn and Re, X is at least one element selected from the group consisting of C, Ge, P, Ga, Sb, In, As and Be, and the factors that define the composition, a, x, y, z, α, β and γ, respectively meet the following conditions: 0≤a<0.5 0.1≤x≤3 10≤y≤20 5≤z≤10 0.1≤α≤5 0≤β≤10 0≤γ≤10, and wherein the Fe-based amorphous alloy ribbon is formed from a molten alloy by a liquid quenching method. 0≤a<0.5 0.1≤x≤3 10≤y≤20 5≤z≤10 0.1≤α≤5 0≤β≤10 0≤γ≤10, and wherein the Fe-based amorphous alloy ribbon is formed from a molten alloy by a liquid quenching method.