In a cooling structure in a clutch having drive plates and driven plates, a lower limit of a width of each of dot grooves is set to a width a at which a flow quantity of lubricating oil passing through each of the dot grooves becomes a minimum flow quantity at which the drive plates and the driven plates can be cooled to a temperature equal to or lower than an upper limit temperature (Tmax) and an upper limit of the width of each of the dot grooves is set to a width b at which an air content in lubricating oil passing through each of the dot grooves becomes a maximum air content at which, according to lubricating oil having the air content, the drive plates and the driven plates can be cooled to a temperature equal to or lower than the upper limit temperature (Tmax).