A semiconductor device comprising: a lead frame; a transistor including a plurality of drain electrode pads (21, 21p, 21Q), a plurality of source electrode pads, and a gate electrode pad on one surface, each of the electrode pads (21, 22, 23) faces a front surface of the lead frame and is connected to the lead frame; and an encapsulation resin that is rectangular and encapsulates the transistor and the lead frame so that part of the lead frame is exposed from a back surface, the back surface being a surface mounted on a circuit substrate, wherein the lead frame includes a drain frame electrically connected to the drain electrode pads (21, 21p, 21Q), a source frame electrically connected to the source electrode pads, and a gate frame electrically connected to the gate electrode pads, the drain frame includes a plurality of drain frame fingers, the drain frame fingers are spaced apart from each other in a first direction, extended in a second direction that is orthogonal to the first direction in a plan view, and connected to the drain electrode pad (21, 21p, 21Q), the source frame includes a plurality of source frame fingers, the source frame fingers are spaced apart from each other in the first direction, extended in the second direction, and connected to the source electrode pad, the drain frame fingers and the source frame fingers are alternately arranged in the first direction and include a portion that overlaps each other as viewed in the first direction, and in a region where the drain frame fingers and the source frame fingers overlap one another as viewed in the first direction, at least either one of the drain frame fingers and the source frame fingers are not exposed from the back surface of the encapsulation resin. a lead frame; a transistor including a plurality of drain electrode pads (21, 21p, 21Q), a plurality of source electrode pads, and a gate electrode pad on one surface, each of the electrode pads (21, 22, 23) faces a front surface of the lead frame and is connected to the lead frame; and an encapsulation resin that is rectangular and encapsulates the transistor and the lead frame so that part of the lead frame is exposed from a back surface, the back surface being a surface mounted on a circuit substrate, wherein the lead frame includes a drain frame electrically connected to the drain electrode pads (21, 21p, 21Q), a source frame electrically connected to the source electrode pads, and a gate frame electrically connected to the gate electrode pads, the drain frame includes a plurality of drain frame fingers, the drain frame fingers are spaced apart from each other in a first direction, extended in a second direction that is orthogonal to the first direction in a plan view, and connected to the drain electrode pad (21, 21p, 21Q), the source frame includes a plurality of source frame fingers, the source frame fingers are spaced apart from each other in the first direction, extended in the second direction, and connected to the source electrode pad, the drain frame fingers and the source frame fingers are alternately arranged in the first direction and include a portion that overlaps each other as viewed in the first direction, and in a region where the drain frame fingers and the source frame fingers overlap one another as viewed in the first direction, at least either one of the drain frame fingers and the source frame fingers are not exposed from the back surface of the encapsulation resin.