An electric pump unit (6, 306) of a transmission, which supplies a hydraulic pressure to the transmission, said electric pump unit (6, 306) comprising: a pump (1, 301) which takes in and discharges oil; and a pump driving electric motor (2, 302) comprising: a motor shaft (12, 312) connected to the pump (1, 301); a motor rotor fixedly provided on the motor shaft (12, 312); and a motor stator (14, 314) disposed around a circumference of the motor rotor, characterized in that the pump (1, 301) and the electric motor (2, 302) are integrated with a lid (5, 305) that tightly closes an opening in a recess portion (4, 304) which is formed on a transmission housing (3, 303) of the transmission and into which the oil is introduced, and are accommodated within the recess portion (4, 304), wherein at least a part of the pump (1, 301) and the electric motor (2, 302) are soaked in the oil, wherein the motor shaft (12, 312) is disposed horizontally and is supported rotatably on the transmission housing (3, 303) or the lid via a bearing (20, 320), and at least a portion of the motor shaft (12, 312) which is situated lower than a center thereof is soaked in the oil, wherein the pump (1, 301) has an oil inlet port and an oil outlet port which are provided in axial end wall which defines a pump compartment within a pump housing (7, 307) so as to communicate with external portions, respectively, and wherein the oil inlet port in the pump housing (7, 307) communicates directly with the external portion, and an oil passage through which the oil is introduced into the oil inlet port from a radially outer side of the pump housing (7, 307) is formed between the pump housing (7, 307) and the motor stator (14, 314). a pump (1, 301) which takes in and discharges oil; and a pump driving electric motor (2, 302) comprising: a motor shaft (12, 312) connected to the pump (1, 301); a motor rotor fixedly provided on the motor shaft (12, 312); and a motor stator (14, 314) disposed around a circumference of the motor rotor, a motor shaft (12, 312) connected to the pump (1, 301); a motor rotor fixedly provided on the motor shaft (12, 312); and a motor stator (14, 314) disposed around a circumference of the motor rotor, characterized in that the pump (1, 301) and the electric motor (2, 302) are integrated with a lid (5, 305) that tightly closes an opening in a recess portion (4, 304) which is formed on a transmission housing (3, 303) of the transmission and into which the oil is introduced, and are accommodated within the recess portion (4, 304), wherein at least a part of the pump (1, 301) and the electric motor (2, 302) are soaked in the oil, wherein the motor shaft (12, 312) is disposed horizontally and is supported rotatably on the transmission housing (3, 303) or the lid via a bearing (20, 320), and at least a portion of the motor shaft (12, 312) which is situated lower than a center thereof is soaked in the oil, wherein the pump (1, 301) has an oil inlet port and an oil outlet port which are provided in axial end wall which defines a pump compartment within a pump housing (7, 307) so as to communicate with external portions, respectively, and wherein the oil inlet port in the pump housing (7, 307) communicates directly with the external portion, and an oil passage through which the oil is introduced into the oil inlet port from a radially outer side of the pump housing (7, 307) is formed between the pump housing (7, 307) and the motor stator (14, 314).