A carrier core material that is formed of ferrite particles in which 48 to 52 mass % of Fe, 16 to 22 mass % of Mn, 1.0 to 3.5 mass % of Mg and 0.05 to 0.5 mass % of Ca are included, wherein when an electrical resistance value with an applied voltage of 500 V in an environment (in an L/L environment) in which a temperature is 10°C and a relative humidity is 35% is R L (Ω · cm), as measured according to the description, and an electrical resistance value with an applied voltage of 500 V in an environment (in an H/H environment) in which a temperature is 30°C and a relative humidity is 70% is R H (Ω · cm), as measured according to the description, formula below is satisfied: 0.1 ≤ log R L − log R H ≤ 0.3 wherein the carrier core material is obtainable by a method comprising a pre-calcination step and a full calcination step, wherein in the pre-calcination step the particle diameter D 90 of the precalcined powder is set to equal or less than 3.5 μm; and the full calcination step includes a temperature rise stage, a heating stage and a cooling stage, wherein the oxygen concentration in a calcined atmosphere is switched to a higher concentration in the second half of the heating stage; the switching of the oxygen concentration is completed by the time the calcination temperature in the cooling stage becomes less than 800°C; and the oxygen concentration in the calcined atmosphere after the switching falls within a range of 4000 to 9000 ppm, wherein a difference in the oxygen concentration before and after the switching falls within a range of 1000 to 4000 ppm. wherein when an electrical resistance value with an applied voltage of 500 V in an environment (in an L/L environment) in which a temperature is 10°C and a relative humidity is 35% is R L (Ω · cm), as measured according to the description, and an electrical resistance value with an applied voltage of 500 V in an environment (in an H/H environment) in which a temperature is 30°C and a relative humidity is 70% is R H (Ω · cm), as measured according to the description, formula below is satisfied: 0.1 ≤ log R L − log R H ≤ 0.3 wherein the carrier core material is obtainable by a method comprising a pre-calcination step and a full calcination step, wherein in the pre-calcination step the particle diameter D 90 of the precalcined powder is set to equal or less than 3.5 μm; and the full calcination step includes a temperature rise stage, a heating stage and a cooling stage, wherein the oxygen concentration in a calcined atmosphere is switched to a higher concentration in the second half of the heating stage; the switching of the oxygen concentration is completed by the time the calcination temperature in the cooling stage becomes less than 800°C; and the oxygen concentration in the calcined atmosphere after the switching falls within a range of 4000 to 9000 ppm, wherein a difference in the oxygen concentration before and after the switching falls within a range of 1000 to 4000 ppm. the oxygen concentration in a calcined atmosphere is switched to a higher concentration in the second half of the heating stage; the switching of the oxygen concentration is completed by the time the calcination temperature in the cooling stage becomes less than 800°C; and the oxygen concentration in the calcined atmosphere after the switching falls within a range of 4000 to 9000 ppm, wherein a difference in the oxygen concentration before and after the switching falls within a range of 1000 to 4000 ppm.