To provide a connection structure of a motor and a pump which is a simple structure, can be assembled easily and is excellent in versatility.

In a connection structure 10, a tip part 14a of a rotational axis 14 of an axial piston type high pressure pump 12 is inserted into a hole part 17 of a hollow axis part 15 provided coaxially with a rotational axis 13 on the tip side of the rotational axis 13 of a motor 11 using a direct current as a driving power source. A key 16 is inserted in between the hole part 17 and the tip part 14a as torque transmission means between the hole part 17 of the hollow axis part 15 of the motor 11 and the tip part 14a of the rotational axis 14 of the high pressure pump 12, inserted into the hole part 17. Connection means 24 connecting a casing 23 of the motor 11 and a casing 20 of the pump 12 is provided to keep the opposed postures of the motor 11 and the pump 12 so that the respective rotational axes 13 and 14 can be connected coaxially.