An image reader is capable of improving the quality of reading a transmission manuscript and of reducing a cost for a luminaire power source compared with the conventional image reader. The image reader comprises a casing (1) having a ceiling (2A) of a transparent manuscript plate (5), the casing (1) further housing therein a read unit (40) provided with a reflection read light source unit (55), a drive mechanism for reciprocally moving the read unit (4) from its home position (HP) and a luminaire power source unit; and a transmission read light source unit (20) provided separately from the casing (1), wherein the transmitted light which has passed the manuscript or the reflected light which has reflected from the manuscript is focused on the receiving surface of a read sensor (48) of the read unit (40) to thereby produce the image thereon, wherein the transmission read light source unit (20) can be connected to the read unit (40) mechanically and electrically by a connecting mechanism having a releasing function, and cooperate with the read unit (40), and wherein the casing (1) has recess portions (6, 6) thereon in the scanning direction of the connecting mechanism, and wherein the luminaire power source unit is provided with an inverter power source (60) which is common to the reflection read light source unit (55) and the transmission read light source unit (20) and a light source unit switching device (61) which can switch the output of the inverter power source (60) to the reflection read light source unit (55) or the transmission read light source unit (20).