To provide an electronic circuit device that can change a characteristic after package sealing and that achieves a reduction in miscellaneous tasks during characteristic setting.The electronic circuit device includes: a burst detecting circuit 7 for detecting, from an input and output terminal 4, a prescribed write activation burst having a length that is larger than or equal to a prescribed time; a signal-pattern detecting circuit 9 for putting a serial interface 8 into an input-enable state in which setting data can be input, when the write activation burst is detected; and a volatile memory 10 and a nonvolatile memory 11 for storing, in the input-enable state, a setting-data signal input from the input and output terminal 4. An operation state of a functional circuit 6 is set in accordance with the setting data written in the volatile memory 10 or the nonvolatile memory 11.