A small-size vehicle having a power unit mounted thereon, and a transmission apparatus incorporated in the power unit, the transmission apparatus including a shift spindle rotatable in response to a gear shifting action, a shift drum rotatable through a shift drum actuating mechanism by rotation of the shift spindle, and a case covering the shift spindle and the shift drum, the transmission apparatus comprising: a shift spindle sensor for detecting rotary movement of the shift spindle; and a shift drum sensor for detecting rotary movement of the shift drum; wherein the shift spindle sensor is mounted in and extends through a through spindle sensor mount hole defined in a spindle sensor mount seat formed on a predetermined portion of the case; the shift drum sensor is mounted in and extends through a through drum sensor mount hole defined in a drum sensor mount seat formed on a predetermined portion of the case; and a direction in which the shift spindle sensor is mounted in and extends through the through spindle sensor mount hole is same as a direction in which the shift drum sensor is mounted in and extends through the through drum sensor mount hole; characterised in that: at a position separated from the shift spindle sensor along a longitudinal direction of the shift spindle, a second shift spindle sensor for detecting rotary movement of the shift spindle is provided on the case, and in that the first-named shift spindle sensor is for detecting rotary movement of the shift spindle in one direction, and the second shift spindle sensor is for detecting rotary movement of the shift spindle in the other direction. a shift spindle sensor for detecting rotary movement of the shift spindle; and a shift drum sensor for detecting rotary movement of the shift drum; wherein the shift spindle sensor is mounted in and extends through a through spindle sensor mount hole defined in a spindle sensor mount seat formed on a predetermined portion of the case; the shift drum sensor is mounted in and extends through a through drum sensor mount hole defined in a drum sensor mount seat formed on a predetermined portion of the case; and a direction in which the shift spindle sensor is mounted in and extends through the through spindle sensor mount hole is same as a direction in which the shift drum sensor is mounted in and extends through the through drum sensor mount hole; characterised in that: at a position separated from the shift spindle sensor along a longitudinal direction of the shift spindle, a second shift spindle sensor for detecting rotary movement of the shift spindle is provided on the case, and in that the first-named shift spindle sensor is for detecting rotary movement of the shift spindle in one direction, and the second shift spindle sensor is for detecting rotary movement of the shift spindle in the other direction. at a position separated from the shift spindle sensor along a longitudinal direction of the shift spindle, a second shift spindle sensor for detecting rotary movement of the shift spindle is provided on the case, and in that the first-named shift spindle sensor is for detecting rotary movement of the shift spindle in one direction, and the second shift spindle sensor is for detecting rotary movement of the shift spindle in the other direction.