An illumination apparatus comprising: an elongate casing; a plurality of light emitting devices serving as a light source; a plurality of lenses disposed to correspond to said plurality of light emitting devices; an elongate, populated board having a front surface and a back surface and populated at said front surface with said plurality of light emitting devices such that at least a portion of said plurality of light emitting devices 32 are aligned in a longitudinal direction of said populated board; a holding member disposed opposite to said front surface of said populated board and holding said plurality of lenses; and a heat radiation member disposed along said populated board opposite to said back surface of said populated board, wherein: said casing includes a bottom wall. a pair of sidewalls erected from a pair of opposite, longer-side ends of said bottom wall. a housing portion defined by said bottom wall and said pair of sidewalls and receiving said plurality of light emitting devices, said populated board, said plurality of lenses, said holding member and said heat radiation member therein, a pair of supporting portions projecting from said pair of sidewalls_( toward said housing portion and supporting a pair of opposite, longer-side ends of said populated board, and a pair of first engagement portions provided at said pair of sidewalls and fitting and securing said holding member to said casing; said holding member includes a plurality of openings associated with said plurality of light emitting devices, a plurality of second engagement portions fitting and securing said plurality of lenses to said holding member in such a manner that said plurality of openings are covered, and a plurality of resilient biasing portions resiliently biasing said pair of longer-side ends of said populated board toward said pair of supporting portions_; and said plurality of resilient biasing portions and said pair of supporting portions pinch and thus secure said populated board, and said populated board and said bottom wall sandwich and thus secure said heat radiation member in contact with said populated board and said bottom wall. an elongate casing; a plurality of light emitting devices serving as a light source; a plurality of lenses disposed to correspond to said plurality of light emitting devices; an elongate, populated board having a front surface and a back surface and populated at said front surface with said plurality of light emitting devices such that at least a portion of said plurality of light emitting devices 32 are aligned in a longitudinal direction of said populated board; a holding member disposed opposite to said front surface of said populated board and holding said plurality of lenses; and a heat radiation member disposed along said populated board opposite to said back surface of said populated board, wherein: said casing includes a bottom wall. a pair of sidewalls erected from a pair of opposite, longer-side ends of said bottom wall. a housing portion defined by said bottom wall and said pair of sidewalls and receiving said plurality of light emitting devices, said populated board, said plurality of lenses, said holding member and said heat radiation member therein, a pair of supporting portions projecting from said pair of sidewalls_( toward said housing portion and supporting a pair of opposite, longer-side ends of said populated board, and a pair of first engagement portions provided at said pair of sidewalls and fitting and securing said holding member to said casing; said holding member includes a plurality of openings associated with said plurality of light emitting devices, a plurality of second engagement portions fitting and securing said plurality of lenses to said holding member in such a manner that said plurality of openings are covered, and a plurality of resilient biasing portions resiliently biasing said pair of longer-side ends of said populated board toward said pair of supporting portions_; and said plurality of resilient biasing portions and said pair of supporting portions pinch and thus secure said populated board, and said populated board and said bottom wall sandwich and thus secure said heat radiation member in contact with said populated board and said bottom wall. said casing includes a bottom wall. a pair of sidewalls erected from a pair of opposite, longer-side ends of said bottom wall. a housing portion defined by said bottom wall and said pair of sidewalls and receiving said plurality of light emitting devices, said populated board, said plurality of lenses, said holding member and said heat radiation member therein, a pair of supporting portions projecting from said pair of sidewalls_( toward said housing portion and supporting a pair of opposite, longer-side ends of said populated board, and a pair of first engagement portions provided at said pair of sidewalls and fitting and securing said holding member to said casing; said holding member includes a plurality of openings associated with said plurality of light emitting devices, a plurality of second engagement portions fitting and securing said plurality of lenses to said holding member in such a manner that said plurality of openings are covered, and a plurality of resilient biasing portions resiliently biasing said pair of longer-side ends of said populated board toward said pair of supporting portions_; and said plurality of resilient biasing portions and said pair of supporting portions pinch and thus secure said populated board, and said populated board and said bottom wall sandwich and thus secure said heat radiation member in contact with said populated board and said bottom wall.