To provide an increased life for a filter by preventing evenly progressing of choking of a filter and to prevent lowering of detecting precision of gas concentration.

An air flow passage 13 through which air flows from an air inlet toward an air outlet 1d is formed in a case 1. A filter 2 is arranged interrupting the air passage 13, and a gas sensor 3 is arranged above the filter 2. In a so formed gas detecting device for a vehicle, root parts 2a are formed in the under surface of the filter and a slope to change the thickness in a vertical direction of the filter 2 is formed. This constitution lengthens a time in which choking of the filter 2 is gradually progressed from a thin section part and the whole of an under surface is choked, i.e., provides an increased life for the filter.