The magnetic field of a star plays a crucial role in the star internal mechanisms, as well as in the interactions with its environment. In particular, the star magnetic field can affect the habitability of the exoplanets it potentially hosts. The objective of this project is to characterize the magnetic field of stars and to understand the star-planet magnetic interaction. Moreover, the analysis of solar-type stars is also useful to shed light on the origin of the solar magnetic field. In the Sun, a relation between the magnetic field and the spots temperature has been observed for sometime. We may naturally expect that the same relation holds in other stars of solar type. Provided this hypothesis is true, the study of starspots gives us information about the magnetic field of the star, such as intensity and characterization of the cycle. Using this, we studied the magnetic field of the solar-type stars CoRoT-2, Kepler-17 and Kepler-63. We applied the method proposed by Silva (2003) that characterizes indirectly the spots (radius, intensity, temperature) by the detection of variations in the light curve of a star caused by the occultation of a spot during a planetary transit. The intensity of the spot may then be converted to temperature by considering that both the spot and the stellar photosphere radiates as a black body. The transit fitting yields spots with mean intensity (relative to the central star intensity Ic) of 0.45±0.24Ic, 0.53±0.19Ic, 0.47±0.16Ic respectively for the three stars. Considering an effective temperature of 5625K (5781K, 5576K) for the stellar photosphere, the mean spot temperature is found to be 4600±700K (5000±600K, 4800±400K). This finally gives access to the mean magnetic field of the stars, 1700±700G (1400±700G, 1600±400G).