A saddled vehicle comprising: a vehicle body frame; a seat disposed behind the vehicle body frame; an onboard storage box (17, 18, 90) removably mounted around the seat; a box stay (30, 31, 93) fixed to a vehicle body side around the seat for supporting the onboard storage box; and a lock mechanism provided between the onboard storage box and the box stay, wherein the lock mechanism includes a lock lever, a key cylinder provided at the lock lever, a cam mounted on the key cylinder and rotated by a key inserted into the key cylinder, a lock plate provided with a first lock hole configured to engage with the cam and previously fixed to the onboard storage box side or the box stay (30, 31, 93) side, and an engaging part provided at the lock lever and configured to engage with the box stay, wherein the lock lever is rotatable between a lock position where the engaging part engages with the box stay (30, 31, 93) and an unlock position where the engaging part disengages from the box stay, the key is rotatable between a lock position on the key cylinder where the cam engages with the first lock hole by being rotated by a predetermined amount and an unlock position on the key cylinder where the cam is rotated by less than the predetermined amount, when the key is operated to establish locking, the key becomes incapable of being removed from the key cylinder unless its rotated position attains the lock position on the key cylinder, characterised in that the lock plate partially includes an extension extending to become spaced apart from the first lock hole and projecting on a rotation path of the cam in the unlock position of the lock lever, and when the lock lever is in the unlock position, the extension projects on the rotation path of the cam to cause the cam to abut on the extension, preventing the key to rotate to reach the lock position on the key cylinder. a vehicle body frame; a seat disposed behind the vehicle body frame; an onboard storage box (17, 18, 90) removably mounted around the seat; a box stay (30, 31, 93) fixed to a vehicle body side around the seat for supporting the onboard storage box; and a lock mechanism provided between the onboard storage box and the box stay, wherein the lock mechanism includes a lock lever, a key cylinder provided at the lock lever, a cam mounted on the key cylinder and rotated by a key inserted into the key cylinder, a lock plate provided with a first lock hole configured to engage with the cam and previously fixed to the onboard storage box side or the box stay (30, 31, 93) side, and an engaging part provided at the lock lever and configured to engage with the box stay, wherein a lock lever, a key cylinder provided at the lock lever, a cam mounted on the key cylinder and rotated by a key inserted into the key cylinder, a lock plate provided with a first lock hole configured to engage with the cam and previously fixed to the onboard storage box side or the box stay (30, 31, 93) side, and an engaging part provided at the lock lever and configured to engage with the box stay, wherein the lock lever is rotatable between a lock position where the engaging part engages with the box stay (30, 31, 93) and an unlock position where the engaging part disengages from the box stay, the key is rotatable between a lock position on the key cylinder where the cam engages with the first lock hole by being rotated by a predetermined amount and an unlock position on the key cylinder where the cam is rotated by less than the predetermined amount, when the key is operated to establish locking, the key becomes incapable of being removed from the key cylinder unless its rotated position attains the lock position on the key cylinder, characterised in that the lock plate partially includes an extension extending to become spaced apart from the first lock hole and projecting on a rotation path of the cam in the unlock position of the lock lever, and when the lock lever is in the unlock position, the extension projects on the rotation path of the cam to cause the cam to abut on the extension, preventing the key to rotate to reach the lock position on the key cylinder.