A portable electronic device includes a casing with a casing pivotal portion and a key device. The key device is installed on the casing and includes a button cap. The button cap includes a cap pivotal portion, a first cap body and a second cap body. The cap pivotal portion is pivoted to the casing pivotal portion. The first cap body extends from the cap pivotal portion along a first axis. The second cap body extends from the first cap body along a second axis. The button cap rotates in an activating direction when the first cap body is applied by an external force along the first axis or when the second cap body is applied by the external force along the second axis.