Abstract: A human-machine interface device and an activating method for a back light unit thereof are provided. The human-machine interface device includes a light sensor, an infrared sensor and a back light unit. The infrared sensor being selectively activated detects whether any part of a user body is within a sensing range when the infrared sensor is activated. When the light sensor detects that the ambient light intensity changes from a higher light intensity to a lower light intensity and the variation from the higher light intensity to the lower light intensity exceeds a predetermined light intensity difference, the infrared sensor is activated for a first period. When the activated infrared sensor detects that the part of a user body is within the sensing range, the back light unit is activated for a second period.