Use of a coil device in keyless entry systems using electronic keys, or electronic theft protection apparatuses, the coil device comprising a coil, and a ferrite core arranged in a magnetic path of the coil, at least part of which is coated with a resin; characterized in that said ferrite core is a Ni ferrite core having an average crystal grain size of 5 μm to 9 μm; and said Ni ferrite core has to meet the following requirements: (a) an initial permeability μi of 450 or more at a frequency of 100 kHz and a temperature of 20°C, (b) temperature-dependent inductance change ratios TLa and TLb of -0.6% to +0.6%, wherein TLa is represented by the formula 1 of TLa = [(LT2 - LT1)/LT1] x 100 (%), and TLb is represented by the formula 2 of TLb = [(LT3 - LT1)/LT1] x 100 (%), wherein LT1 is inductance in a compression-free state at a frequency of 100 kHz and 20°C, LT2 is inductance in a compression-free state at a frequency of 100 kHz and -40°C, and LT3 is inductance in a compression-free state at a frequency of 100 kHz and 80°C, and (c) stress-dependent inductance change ratios PLa and PLb of -0.6% to +0.6%, wherein PLa is represented by the formula 3 of PLa = [(LP2 - LP1)/LP1] x 100 (%), and PLb is represented by the formula 4 of PLb = [(LT1 - LP1)/LP1] x 100 (%), wherein LP1 is inductance when monoaxially compressed with a surface pressure of 10 MPa at a frequency of 100 kHz and 20°C, LP2 is inductance when monoaxially compressed with a surface pressure of 20 MPa at a frequency of 100 kHz and 20°C, and LT1 is inductance in a compression-free state at a frequency of 100 kHz and 20°C, (d) both of the sum of TLa and PLa and the sum of TLb and PLb being more than -1.0% and less than + 1.0%; and characterised in that said Ni ferrite core has a composition consisting of 47.5% to 48.4% by mol of Fe 2 O 3, 25.0% to 30.5% by mol of ZnO, and 7.0% to 11.5% by mol of CuO, the balance being NiO and inevitable impurities containing 0.1 parts or less by mass of Ca calculated as CaO, 0.1 parts or less by mass of Si calculated as SiO 2, and 0.05 parts or less by mass of at least selected from the group consisting of Na, S, Cl, P, Mn, Cr and B, per 100 parts by mass in total of Fe 2 O 3, ZnO, CuO and NiO. said ferrite core is a Ni ferrite core having an average crystal grain size of 5 μm to 9 μm; and said Ni ferrite core has to meet the following requirements: (a) an initial permeability μi of 450 or more at a frequency of 100 kHz and a temperature of 20°C, (b) temperature-dependent inductance change ratios TLa and TLb of -0.6% to +0.6%, wherein TLa is represented by the formula 1 of TLa = [(LT2 - LT1)/LT1] x 100 (%), and TLb is represented by the formula 2 of TLb = [(LT3 - LT1)/LT1] x 100 (%), wherein LT1 is inductance in a compression-free state at a frequency of 100 kHz and 20°C, LT2 is inductance in a compression-free state at a frequency of 100 kHz and -40°C, and LT3 is inductance in a compression-free state at a frequency of 100 kHz and 80°C, and (c) stress-dependent inductance change ratios PLa and PLb of -0.6% to +0.6%, wherein PLa is represented by the formula 3 of PLa = [(LP2 - LP1)/LP1] x 100 (%), and PLb is represented by the formula 4 of PLb = [(LT1 - LP1)/LP1] x 100 (%), wherein LP1 is inductance when monoaxially compressed with a surface pressure of 10 MPa at a frequency of 100 kHz and 20°C, LP2 is inductance when monoaxially compressed with a surface pressure of 20 MPa at a frequency of 100 kHz and 20°C, and LT1 is inductance in a compression-free state at a frequency of 100 kHz and 20°C, (d) both of the sum of TLa and PLa and the sum of TLb and PLb being more than -1.0% and less than + 1.0%; and characterised in that (a) an initial permeability μi of 450 or more at a frequency of 100 kHz and a temperature of 20°C, (b) temperature-dependent inductance change ratios TLa and TLb of -0.6% to +0.6%, wherein TLa is represented by the formula 1 of TLa = [(LT2 - LT1)/LT1] x 100 (%), and TLb is represented by the formula 2 of TLb = [(LT3 - LT1)/LT1] x 100 (%), wherein LT1 is inductance in a compression-free state at a frequency of 100 kHz and 20°C, LT2 is inductance in a compression-free state at a frequency of 100 kHz and -40°C, and LT3 is inductance in a compression-free state at a frequency of 100 kHz and 80°C, and (c) stress-dependent inductance change ratios PLa and PLb of -0.6% to +0.6%, wherein PLa is represented by the formula 3 of PLa = [(LP2 - LP1)/LP1] x 100 (%), and PLb is represented by the formula 4 of PLb = [(LT1 - LP1)/LP1] x 100 (%), wherein LP1 is inductance when monoaxially compressed with a surface pressure of 10 MPa at a frequency of 100 kHz and 20°C, LP2 is inductance when monoaxially compressed with a surface pressure of 20 MPa at a frequency of 100 kHz and 20°C, and LT1 is inductance in a compression-free state at a frequency of 100 kHz and 20°C, (d) both of the sum of TLa and PLa and the sum of TLb and PLb being more than -1.0% and less than + 1.0%; and