To investigate electronic states under extreme conditions such as low temperatures, high magnetic fields, and high pressures, are of special importance because electronic systems are strongly affected by external perturbations since they have quantum mechanical degrees of freedom. Therefore, developments of high-resolution physical property measurements in such extreme conditions are situated in the essential position for precise discussion of physical properties. We have been engaged in developments and improvements of calorimetry technique and high-resolution calorimeters to obtain precise thermodynamic information utilized for cultivated discussions of electronic states. In this work, some relaxation calorimeters were designed for low-temperature heat capacity measurements with extremely small amounts of samples with external magnetic fields whose directions against crystal axes are precisely controlled. Employing these calorimeters, we have performed thermodynamic discussion on the low-temperature electronic and magnetic states of organic conductors. Because they have a rich variety of electronic phases originating from the molecular degrees of freedom, detection of phase transitions and low energy excitations by heat capacity can give entropic information to clarify the mechanism of the phenomena. Our high-resolution calorimeters have succeeded to provide some novel insights into the electronic states formed by π-electrons. We also introduce the process of the developments of the calorimeters and show our recent results of heat capacity measurements of some organic salts.