A vehicle door latch device, comprising: a base adapted to be fixed to a door; a latch pivotally supported on a surface side of the base, and configured to be rotatable between a fully latched position and an open position in association with opening and closing of the door, the latch having a cam portion on an outer periphery thereof; a locking and unlocking mechanism (18, 19, 20) configured to be switchable, based on operation of a locking and unlocking operation unit provided in the door, to an unlocked position where opening operation on an operation handle provided on the door is validated and to a locked position where the opening operation on the operation handle is invalidated; a detection lever pivotally supported on a reverse side of the base by a shaft parallel with an axial direction of the latch, the detection lever having a detecting portion, which penetrates through an elongated hole provided in the base into the surface side and slides along the cam portion in association with rotation of the latch, and a cam surface provided on an opposite side of the detecting portion across the shaft, the detection lever being configured to be rotatable, by the detecting portion sliding along the cam portion in association with rotation of the latch, to a position corresponding to a rotational position of the latch; and a detection switch provided on the reverse side of the base and configured to be able to detect open and closed states of the door by being turned ON and OFF by coming into contact with and separating from the cam surface in association with rotation of the detection lever, characterized in that the detection lever further has a blocking portion that allows switch-over to the locked position of the locking and unlocking mechanism (18, 19, 20) by retracting outside a movement track of the locking and unlocking mechanism (18, 19, 20) when the blocking portion is at a first rotational position corresponding to the fully latched position of the latch, and blocks movement of the locking and unlocking mechanism (18, 19, 20) to the locked position by advancing into the movement track of the locking and unlocking mechanism (18, 19, 20) when the blocking portion is at a second rotational position corresponding to a position other than the fully latched position of the latch, the cam surface of the detection lever has a shape that bulges out in the axial direction, and the detection switch is arranged to face the cam surface. a base adapted to be fixed to a door; a latch pivotally supported on a surface side of the base, and configured to be rotatable between a fully latched position and an open position in association with opening and closing of the door, the latch having a cam portion on an outer periphery thereof; a locking and unlocking mechanism (18, 19, 20) configured to be switchable, based on operation of a locking and unlocking operation unit provided in the door, to an unlocked position where opening operation on an operation handle provided on the door is validated and to a locked position where the opening operation on the operation handle is invalidated; a detection lever pivotally supported on a reverse side of the base by a shaft parallel with an axial direction of the latch, the detection lever having a detecting portion, which penetrates through an elongated hole provided in the base into the surface side and slides along the cam portion in association with rotation of the latch, and a cam surface provided on an opposite side of the detecting portion across the shaft, the detection lever being configured to be rotatable, by the detecting portion sliding along the cam portion in association with rotation of the latch, to a position corresponding to a rotational position of the latch; and a detection switch provided on the reverse side of the base and configured to be able to detect open and closed states of the door by being turned ON and OFF by coming into contact with and separating from the cam surface in association with rotation of the detection lever, characterized in that the detection lever further has a blocking portion that allows switch-over to the locked position of the locking and unlocking mechanism (18, 19, 20) by retracting outside a movement track of the locking and unlocking mechanism (18, 19, 20) when the blocking portion is at a first rotational position corresponding to the fully latched position of the latch, and blocks movement of the locking and unlocking mechanism (18, 19, 20) to the locked position by advancing into the movement track of the locking and unlocking mechanism (18, 19, 20) when the blocking portion is at a second rotational position corresponding to a position other than the fully latched position of the latch, the cam surface of the detection lever has a shape that bulges out in the axial direction, and the detection switch is arranged to face the cam surface.