A spindle comprising: a housing; a bearing housed in the housing; a main shaft rotatably supported by the bearing, the main shaft being provided with a through hole passing through the main shaft in an axial direction of the main shaft, the axial direction being an up-down direction, the main shaft including a first end located on a lower side in the axial direction and a second end located on an upper side in the axial direction; a collet chuck extending into the through hole and being movable in the axial direction, the collet chuck including a first collet end located on the lower side in the axial direction adjacent to the first end of the main shaft and a second collet end located on the upper side in the axial direction, the first collet end of the collet chuck being able to grip a machining tool to machine a workpiece; a draw bar extending into the through hole and being movable in the axial direction, the draw bar including a first bar end located on the lower side in the axial direction and a second bar end located on the upper side in the axial direction adjacent to the second end of the main shaft the first bar end of the draw bar being connected to the second collet end of the collet chuck; a first securer secured to the main shaft and holding the bearing from below; and a second securer secured to the housing, holding the bearing from below, and located outward of the first securer; wherein the housing includes: an air inlet which is located above the bearing and through which air is introduced into the housing; an air passage located below the bearing such that the air introduced into the housing through the air inlet is capable of flowing below the bearing; and an air outlet which is located below the bearing and through which the air flowing below the bearing is capable of being discharged out of the housing; the air passage includes: a first portion located below the bearing; and a second portion located laterally of the first securer; and the air outlet is located below the second portion; the air flowing through the first portion being able to pass along a surface of the bearing; the first portion of the air passage is defined by the bearing and a recess in the second securer; and the second portion of the air passage is defined by the first securer and the second securer. a housing; a bearing housed in the housing; a main shaft rotatably supported by the bearing, the main shaft being provided with a through hole passing through the main shaft in an axial direction of the main shaft, the axial direction being an up-down direction, the main shaft including a first end located on a lower side in the axial direction and a second end located on an upper side in the axial direction; a collet chuck extending into the through hole and being movable in the axial direction, the collet chuck including a first collet end located on the lower side in the axial direction adjacent to the first end of the main shaft and a second collet end located on the upper side in the axial direction, the first collet end of the collet chuck being able to grip a machining tool to machine a workpiece; a draw bar extending into the through hole and being movable in the axial direction, the draw bar including a first bar end located on the lower side in the axial direction and a second bar end located on the upper side in the axial direction adjacent to the second end of the main shaft the first bar end of the draw bar being connected to the second collet end of the collet chuck; a first securer secured to the main shaft and holding the bearing from below; and a second securer secured to the housing, holding the bearing from below, and located outward of the first securer; wherein the housing includes: an air inlet which is located above the bearing and through which air is introduced into the housing; an air passage located below the bearing such that the air introduced into the housing through the air inlet is capable of flowing below the bearing; and an air outlet which is located below the bearing and through which the air flowing below the bearing is capable of being discharged out of the housing; an air inlet which is located above the bearing and through which air is introduced into the housing; an air passage located below the bearing such that the air introduced into the housing through the air inlet is capable of flowing below the bearing; and an air outlet which is located below the bearing and through which the air flowing below the bearing is capable of being discharged out of the housing; the air passage includes: a first portion located below the bearing; and a second portion located laterally of the first securer; and a first portion located below the bearing; and a second portion located laterally of the first securer; and the air outlet is located below the second portion; the air flowing through the first portion being able to pass along a surface of the bearing; the first portion of the air passage is defined by the bearing and a recess in the second securer; and the second portion of the air passage is defined by the first securer and the second securer.