An electronic circuit device 1 arranged with a first substrate 11, a core 33 (magnetic body) mounted on the first substrate 11, a resin sealing body 17 which covers the first substrate 11 and the core 33, and a curable type stress relieving material 35 which reduces stress applied to the core 33 by the resin sealing body 17 is arranged within the resin sealing body 33 from the side surface periphery of the core 33 across to the first substrate 11.