A connecting structure comprising a terminal fitting retained into a connector housing and a device casing attached onto a substrate, the terminal fitting being electrically connected to an external input contact from the circuit substrate when the housing is fitted into the device casing, the connecting structure comprising: a contact portion that is provided for butting so as to protrude in a front end portion of the terminal fitting and that is received and retained in the connector housing so as to protrude from a front end of the connector housing; and a housing engagement portion that is formed in an outer wall portion of the device casing and that positions the connector housing so that the contact portion protrudes into the device casing, wherein a flat plate-like conductor of the circuit substrate serving as the external input contact is disposed in the device casing so that a surface of the flat plate-like conductor abuts against the contact portion protruding from the front end of the connector housing positioned by the housing engagement portion, wherein the contact portion of the terminal fitting is a part of a plate spring piece (21 a) formed integrally with the terminal fitting, and is provided so that the contact portion can be elastically displaced toward a rear end portion of the terminal fitting due to elastic deformation of the plate spring piece (21 a) when the contact portion abuts against the flat plate-like conductor, the housing engagement portion of the device casing includes a hood portion and a housing lock portion, the hood portion being formed into a cylindrical structure fitted to an outer periphery of the connector housing so as to restrict an insertion direction of the connector housing in a direction substantially perpendicular to the circuit substrate, characterized in that the housing lock portion is a protrusion which is provided on the hood portion of the housing engagement portion, this protrusion being engaged with a lock piece provided on an outer surface of the connector housing to fix the connector housing to the device casing when the contact portion of the terminal fitting abuts against the flat plate-like conductor with a set pressure due to insertion of the connector housing into the hood portion; and a pair of elongated ridge portions extending along the insertion direction of the connector housing and formed on the outer side surface of the connector housing, the lock piece is arranged between the pair of elongated ridge portions. a contact portion that is provided for butting so as to protrude in a front end portion of the terminal fitting and that is received and retained in the connector housing so as to protrude from a front end of the connector housing; and a housing engagement portion that is formed in an outer wall portion of the device casing and that positions the connector housing so that the contact portion protrudes into the device casing, wherein a flat plate-like conductor of the circuit substrate serving as the external input contact is disposed in the device casing so that a surface of the flat plate-like conductor abuts against the contact portion protruding from the front end of the connector housing positioned by the housing engagement portion, wherein the contact portion of the terminal fitting is a part of a plate spring piece (21 a) formed integrally with the terminal fitting, and is provided so that the contact portion can be elastically displaced toward a rear end portion of the terminal fitting due to elastic deformation of the plate spring piece (21 a) when the contact portion abuts against the flat plate-like conductor, the housing engagement portion of the device casing includes a hood portion and a housing lock portion, the hood portion being formed into a cylindrical structure fitted to an outer periphery of the connector housing so as to restrict an insertion direction of the connector housing in a direction substantially perpendicular to the circuit substrate, characterized in that the housing lock portion is a protrusion which is provided on the hood portion of the housing engagement portion, this protrusion being engaged with a lock piece provided on an outer surface of the connector housing to fix the connector housing to the device casing when the contact portion of the terminal fitting abuts against the flat plate-like conductor with a set pressure due to insertion of the connector housing into the hood portion; and a pair of elongated ridge portions extending along the insertion direction of the connector housing and formed on the outer side surface of the connector housing, the lock piece is arranged between the pair of elongated ridge portions.