To secure the time required till starter ON from ignition ON long to some extent, in a vehicular power source device and a two-stage insertion key cylinder.

This two-stage insertion type key cylinder is provided with a key hole for inserting a key, an IG operation mechanism capable of ignition ON operation in a state of inserting the key into the key hole up to a first stage position, and a starter operation mechanism capable of starter ON operation in a state of further inserting the key into the key hole up to a second stage position after the ignition ON operation. An ignition switch is turned on when the ignition ON operation is performed in response to the IG operation mechanism, and afterwards, a starter switch is turned on when the starter ON operation is performed in response to the starter operation mechanism.