A completely-in-canal hearing aid, comprising a housing, a receiver, a chip, a battery, a flexible circuit board and a microphone, wherein: the housing comprises a front section and a rear section; the receiver and the chip are located in the front section; the battery is located in the rear section; the rear section has two opposite surfaces (10a, 10b) and two opposite side surfaces (10c, 10d); the front section has a first central axis parallel to the length thereof; the rear section has a second central axis located between the two opposite surfaces (10a, 10b) and between the two opposite side surfaces (10c, 10d); the first central axis and the second central axis are different straight line s; the two opposite surfaces (10a, 10b) of the rear section are planes; the housing is divided into a first housing body and a second housing body; the first housing body and the second housing body each have one of the opposite side surfaces (10c, 10d), the opposite side surface of the first housing body is a first side surface, the opposite side surface of the second housing body is a second side surface, the first side surface is shown as a part of a cylindrical side surface, and the second side surface is shown as another part of the cylindrical side surface; the first housing body comprises a first front section and a first rear section connected to each other; the second housing body comprises a second front section and a second rear section connected to each other; the receiver and the chip are located in the first front section and the second front section; the battery is located in the first rear section and the second rear section; the battery is a button battery; and a first end plate used for fixing the microphone, characterised in that the flexible circuit board has an annular portion bent into an annular shape, and the annular portion surrounds the peripheral side of the battery, wherein an outer side surface of the annular portion has pads; the flexible circuit board has a device portion located on a front side of the annular portion, and the device portion is used for installing electronic devices and is used for electrically connecting the receiver; the flexible circuit board has a chip portion connected to a bottom end of the annular portion, and the chip portion is used for binding a chip; and a second end plate used for fixing a switch; wherein a first connecting section is connected between the first end plate and the annular portion, and a second connecting section is connected between the second end plate and the annular portion; the first connecting section and the second connecting section intersect each other, and the intersection becomes an intersection position of the annular portion; wherein the annular portion is divided into two sections by the device portion and the intersection position, and an outer side surface of each section has two pads. the housing comprises a front section and a rear section; the receiver and the chip are located in the front section; the battery is located in the rear section; the rear section has two opposite surfaces (10a, 10b) and two opposite side surfaces (10c, 10d); the front section has a first central axis parallel to the length thereof; the rear section has a second central axis located between the two opposite surfaces (10a, 10b) and between the two opposite side surfaces (10c, 10d); the first central axis and the second central axis are different straight line s; the two opposite surfaces (10a, 10b) of the rear section are planes; the housing is divided into a first housing body and a second housing body; the first housing body and the second housing body each have one of the opposite side surfaces (10c, 10d), the opposite side surface of the first housing body is a first side surface, the opposite side surface of the second housing body is a second side surface, the first side surface is shown as a part of a cylindrical side surface, and the second side surface is shown as another part of the cylindrical side surface; the first housing body comprises a first front section and a first rear section connected to each other; the second housing body comprises a second front section and a second rear section connected to each other; the receiver and the chip are located in the first front section and the second front section; the battery is located in the first rear section and the second rear section; the battery is a button battery; and a first end plate used for fixing the microphone, characterised in that the flexible circuit board has an annular portion bent into an annular shape, and the annular portion surrounds the peripheral side of the battery, wherein an outer side surface of the annular portion has pads; the flexible circuit board has a device portion located on a front side of the annular portion, and the device portion is used for installing electronic devices and is used for electrically connecting the receiver; the flexible circuit board has a chip portion connected to a bottom end of the annular portion, and the chip portion is used for binding a chip; and a second end plate used for fixing a switch; wherein a first connecting section is connected between the first end plate and the annular portion, and a second connecting section is connected between the second end plate and the annular portion; the first connecting section and the second connecting section intersect each other, and the intersection becomes an intersection position of the annular portion; wherein the annular portion is divided into two sections by the device portion and the intersection position, and an outer side surface of each section has two pads.