A sensor having a detecting part for detecting an environmental value by a smoke flow or a thermal flow flowing from outside, a controlling part for discriminating abnormality in circumference environment based on the environmental value detected by the detecting part, and a sounding body for alarming based on abnormality discrimination by the controlling part, the sensor comprising: a housing having a base part to be provided on an attachment surface, a ring-like side wall projecting out of the base part, and a disk-like top panel covering an end of the ring-like side wall; a separating plate by which the housing is defined as two divided spaces of a first space on a side of the base part and a second space on a side of the disk-like top panel between the base part and the disk-like top panel; a first opening for opening the first space and adapted to introduce a smoke flow or a thermal flow from outside into the housing, the first opening being formed in a position corresponding to the first space on a side face of the housing; a sound aperture penetrating into the second space in an area of the disk-like top panel which is opposite to the sounding body, wherein the detecting part is provided in the first space whereas the sounding body is provided in the second space. characterized by further comprising a second opening for opening the second space (l1b) and adapted to reduce the air resistance in the second space (l1b) when the sounding body outputs an alarm, the second opening being formed in a position corresponding to the second space (l1b) on the side face of the housing. the sensor comprising: a housing having a base part to be provided on an attachment surface, a ring-like side wall projecting out of the base part, and a disk-like top panel covering an end of the ring-like side wall; a separating plate by which the housing is defined as two divided spaces of a first space on a side of the base part and a second space on a side of the disk-like top panel between the base part and the disk-like top panel; a housing having a base part to be provided on an attachment surface, a ring-like side wall projecting out of the base part, and a disk-like top panel covering an end of the ring-like side wall; a separating plate by which the housing is defined as two divided spaces of a first space on a side of the base part and a second space on a side of the disk-like top panel between the base part and the disk-like top panel; a first opening for opening the first space and adapted to introduce a smoke flow or a thermal flow from outside into the housing, the first opening being formed in a position corresponding to the first space on a side face of the housing; a sound aperture penetrating into the second space in an area of the disk-like top panel which is opposite to the sounding body, wherein the detecting part is provided in the first space whereas the sounding body is provided in the second space. a sound aperture penetrating into the second space in an area of the disk-like top panel which is opposite to the sounding body, wherein the detecting part is provided in the first space whereas the sounding body is provided in the second space. characterized by further comprising a second opening for opening the second space (l1b) and adapted to reduce the air resistance in the second space (l1b) when the sounding body outputs an alarm, the second opening being formed in a position corresponding to the second space (l1b) on the side face of the housing.