NOVELTY - The reactor (1) has a main portion which is provided with a coil (10). The coil is accommodated in the case (30). The casting resin (40) is filled inside the case. The coil is integrally fixed to bottom face (31) of case. The specific gravity of casting resin is set as 1.85 g/cm3 or less, while the heat conductivity of casting resin is set as 0.7 W/masteriskK or less. The volume resistivity of the casting resin is more than 2asterisk1010 ohm-cm. The casting resin is provided with 70 mass % or less filler. The average particle diameter of filler is set as 100 micrometers or less.    USE - Reactor for accommodating casting resin (Claimed), used for power converter devices such as direct current-direct current converter mounted in vehicle such as hybrid vehicle, electric automatic air vehicle and fuel cell electric vehicle.    ADVANTAGE - Since the specific gravity of casting resin is set as 1.85 g/cm3 or less, the crack of casting resin which comes out in small quantities and originates in presence of filler can be suppressed even if the filler is not contained in casting resin. The coil is integrally fixed to the bottom face of a case, so that the thermal radiation from coil is efficiently performed via the case bottom face. The required heat dissipation is secured even if the heat conductivity of casting resin is 0.7 W/masteriskK or less. The suppression of the heat dissipation of a coil and the crack of a casting resin is compatible. The insulation between the coil turn is fully maintained when the volume resistivity of casting resin is more than 2 x1010 ohm-cm. The heat conductivity of casting resin is improved by the filler and thermal radiation of coil occurs easily via casting resin when the filler is contained. The casting resin easily permeates in the space between the coil turns if the average particle diameter of filler is 100 micrometers or less. The casting resin easily infiltrates into the constriction portion when the casting resin is a curable resin having a viscosity of 5000 mPaasterisks or less in 50 degrees c. The high casting property is obtained. The reactor in which the suppression of heat dissipation of coil and crack of casting resin is compatible can be obtained.    DETAILED DESCRIPTION - The casting resin is curable resin having a viscosity of 5000 mPaasterisks or less in 50 degrees c.    DESCRIPTION OF DRAWING(S) - The drawing shows a cross-sectional view of the reactor.    Reactor (1)    Coil (10)    Case (30)    Bottom face of case (31)    Casting resin (40)