To provide a rotary slide type mobile terminal unit whose upper casing and lower casing are connected by rotary slide type connection, and which is capable of securing excellent antenna characteristics in both of the state of opening both casings and the state of closing them.

The mobile terminal unit 10 includes a first casing 1 loaded with a radio circuit 4b, a first ground pattern 4a and an antenna element 5, and a second casing 2 having a second ground pattern 7a. The antenna element 5 configures a radiation element together with the first ground pattern 4a. The second ground pattern 7a forms a dipole antenna together with the first ground pattern 4a through capacitive coupling with the antenna element 5 at an open position at which the second ground pattern 7a does not planarly overlap with the first casing 1. Also, the capacitive coupling of the second ground pattern 7a with the antenna element 5 is released at a closed position at which the second ground pattern 7a planarly overlaps with the first ground pattern 4a.