I have an outbreak on my inner thighs . what is the cause of it ?

Hard to say. Skin issues usually require a direct exam if a detailed description does not point to a cause. Here we don't have a description, just a location. You are welcome to start over & provide more information to work with.

Is it itchy? Raised? Do you wear tight jeans? A nonitchy rash there is often the result of friction due to rubbing thighs together. Occasionally it's a reaction to the clothing material or a detergent. A physician or dermatologist would have to examine you in order to make a diagnosis and advise you.