This has never been definitively proven. Studies have shown that malignant melanoma is more prevalent on parts of the body not normally exposed to sunlight, suggesting that these are the areas that have to deal with intermittent over exposure and are at most risk. It is therefore important to reduce the risk of over-exposure.
However, this is as true of sunlight and using sunbeds.

Outdoor tanning exposes the body to different levels of UV in an uncontrolled way, and using sunscreen is only effective when used properly. Recent studies have shown that sunscreen does not provide the protection we expect because we don’t use enough often enough.
It follows that the use of a sunbed, in moderation, can be safer than tanning outdoors because the exposure can be controlled.

UV light has many essential health benefits, for our bodies and mental health too. In fact we need it to survive. Over exposure can be detrimental to health including contributing to skin cancer if it is abused. To stay safe never over use a sunbed and never burn, just as in daylight.

Check your skin regularly and if you notice any changes consult your GP.